Accessibility glossary: terms in plain English

Terms are grouped by topic and listed alphabetically within each group, and the A to Z list below links straight to any term. A definition in quotation marks is W3C's own wording, taken from the WCAG 2.2 glossary or the WAI-ARIA specification, and the entry links to it. Most entries without quotation marks are our plain-language explanation, with the source it rests on linked beside it. Laws and standards define some terms for their own purposes, so check the source text for exact legal wording.

• Standards and conformance
• Laws, regulations and reports
• People and assistive technology
• Code and structure
• Images, media and text alternatives
• Visual design and interaction
• Testing, tools and vendors

All terms, A to Z

A: Accessibility audit, Accessibility overlay, Accessibility statement, Accessibility tree, Accessible Canada Act, Accessible name, ACR, ADA, ADA Title II, ADA Title III, Alt text, AODA, ARIA, Assistive technology, Audio description, Automated testing

B: BFSG

C: Captions, Color vision deficiency (color blindness), Complex image, Conformance, Conformance level (A, AA, AAA), Contrast ratio, CSS pixel

D: Decorative image, Demand letter

E: EN 301 549, Equality Act 2010, European Accessibility Act (EAA)

F: False positive, Flash, Focus, Focus indicator, Focus order, Functional image

G: Guideline

H: Heading structure

I: Image of text

K: Keyboard accessible, Keyboard trap

L: Label, Landmark, Language of the page, Large-scale text, Level AA, Link purpose, Live region

M: Manual testing, Meaningful sequence (reading order), Microenterprise, Monitoring

N: Non-text content, Non-text contrast

P: POUR (the four principles), Programmatically determined, PSBAR

R: Reflow, Role, Rule-based scanner

S: Screen magnifier, Screen reader, Section 508, Semantic HTML, Situational limitation, Skip link, Status message, Success criterion

T: Target size, Text alternative, Transcript

U: Use of color, User agent

V: VPAT

W: W3C and WAI, WCAG, WCAG 2.2, WCAG 3.0, WCAG version (2.0, 2.1, 2.2), Web accessibility, Web Accessibility Directive

Standards and conformance

Accessibility statement

A public page that says how accessible a site is, what is known not to work and how to report a problem. The EU Web Accessibility Directive (Article 7) requires public sector bodies to publish one with a feedback mechanism, and UK public sector rules require one too. See our accessibility statement guide, W3C's statement guidance and the statement generator.

Conformance

W3C defines conformance as "satisfying all the requirements of a given standard, guideline or specification." A page either conforms at a stated level or it does not; progress toward a level is worth reporting but is not conformance. See the WCAG 2.2 glossary and WCAG levels A, AA and AAA.

Conformance level (A, AA, AAA)

WCAG success criteria sit at three levels: A (the minimum), AA and AAA (the highest). To conform at Level AA, a page satisfies all Level A and Level AA success criteria. See WCAG levels A, AA and AAA and W3C's conformance requirements.

Guideline

One of the 13 goals WCAG sets beneath its four principles. W3C says the guidelines are not testable; the success criteria under them are. See WCAG 2.2 explained and W3C's layers of guidance.

Level AA

The conformance level most accessibility laws name. WCAG 2.2 has 55 success criteria at Levels A and AA together (31 at A and 24 at AA, counted by GotAlt from the W3C Recommendation), and laws often name an older WCAG version at AA. See which WCAG version a law requires and W3C's conformance requirements.

POUR (the four principles)

The four WCAG principles: perceivable, operable, understandable and robust. Every success criterion sits under one of them. See the four principles in plain English and W3C's layers of guidance.

Success criterion

A testable requirement in WCAG. W3C writes each one as a statement that will be "either true or false" when specific content is tested against it, and gives each a level (A, AA or AAA). WCAG 2.2 has 86. See the WCAG 2.2 criteria list and W3C's introduction to Understanding WCAG.

W3C and WAI

The World Wide Web Consortium (W3C) is the standards body that publishes WCAG and WAI-ARIA. Its Web Accessibility Initiative (WAI) is the part that works on accessibility and publishes guidance such as the introduction to web accessibility. See also our plain-language guide to web accessibility.

WCAG

The Web Content Accessibility Guidelines, published by W3C. W3C says the WCAG documents "explain how to make web content more accessible to people with disabilities." See WCAG 2.2 explained in plain English and W3C's WCAG overview.

WCAG 2.2

The current version of WCAG, a W3C Recommendation of October 5, 2023, updated on December 12, 2024. It adds nine success criteria (2.4.11, 2.4.12, 2.4.13, 2.5.7, 2.5.8, 3.2.6, 3.3.7, 3.3.8 and 3.3.9) and removes 4.1.1 Parsing, and its October 2023 version is exactly the same as the ISO standard ISO/IEC 40500:2025. See what is new in WCAG 2.2, the WCAG 2.2 Recommendation and W3C's what's new page.

WCAG 3.0

A W3C Working Draft for a future set of guidelines. W3C says WCAG 3 will not supersede WCAG 2 and that WCAG 2 will not be deprecated for several years after WCAG 3 is finalized, and no law we cover references it. See WCAG 3.0: status and what to do now and W3C's WCAG 3 introduction.

WCAG version (2.0, 2.1, 2.2)

WCAG 2.0 was published on December 11, 2008, WCAG 2.1 on June 5, 2018 and WCAG 2.2 on October 5, 2023. W3C calls the versions backwards compatible: content that conforms to WCAG 2.2 also conforms to 2.1 and 2.0, except that 2.2 removed 4.1.1 Parsing as obsolete. See which WCAG version the law requires and W3C's WCAG overview.

Web accessibility

W3C says it means that websites, tools and technologies are "designed and developed so that people with disabilities can use them." It covers auditory, cognitive, neurological, physical, speech and visual disabilities, and it also helps people without disabilities, such as older people and anyone with a situational limit like bright sunlight. See what is web accessibility and W3C's introduction to web accessibility.

Laws, regulations and reports

Entries on laws are orientation, not legal advice. Our accessibility laws overview gives the primary source and a last-verified date for each jurisdiction.

Accessible Canada Act

Canada's federal accessibility law (S.C. 2019, c. 10) for federally regulated organizations. Its Digital Technologies Phase 1 rules point to CAN/ASC-EN 301 549:2024 and cover the public sector and private organizations with 100 or more employees. See accessibility law in Canada and the Government of Canada guidance.

ACR

Accessibility Conformance Report. The Information Technology Industry Council (ITI) says a version of the VPAT completed for a specific product is an ACR, so it reports test results for that product. See VPAT and ACR explained and ITI's VPAT page.

ADA

The Americans with Disabilities Act, a US federal civil rights law that prohibits discrimination against people with disabilities. Title II covers state and local governments and Title III covers businesses open to the public. See ADA website compliance for businesses and the ADA.gov introduction.

ADA Title II

The part of the ADA that covers state and local governments, including special district governments. DOJ's web rule requires WCAG 2.1 Level AA, with compliance dates of April 26, 2027 (population 50,000 or more) and April 26, 2028 (under 50,000 and special districts). See the ADA Title II deadline, Title II for local government websites and the ADA.gov web rule fact sheet.

ADA Title III

The part of the ADA that covers businesses open to the public. There is no Title III web regulation: DOJ guidance of March 18, 2022 says the ADA applies to the web offerings of public accommodations and that existing technical standards, including WCAG, provide "helpful guidance." See ADA website compliance and ADA website lawsuits.

AODA

The Accessibility for Ontarians with Disabilities Act. Under O. Reg. 191/11, section 14, designated public sector organizations and businesses and non-profits with 50 or more employees in Ontario must meet WCAG 2.0 AA, excluding 1.2.4 and 1.2.5, as they have since January 1, 2021. See accessibility law in Canada and Ontario.ca.

BFSG

Germany's Barrierefreiheitsstärkungsgesetz, the national law that implements the European Accessibility Act. Service microenterprises are exempt (section 3(3)) and fines go up to EUR 100,000 (section 37). See Germany's BFSG and the law text at Gesetze im Internet.

Demand letter

In accessibility, a letter saying that a website has barriers that keep people with disabilities from using it and asking the site owner to act. It is not a lawsuit, though a lawsuit may follow. See what to check first after an ADA website demand letter.

EN 301 549

The European standard on accessibility requirements for ICT products and services. Version 3.2.1 is the current referenced standard and is aligned to WCAG 2.1 Level AA; version 4.1.1, published by ETSI on September 2, 2026 and aligned to WCAG 2.2, is not yet cited in the EU Official Journal, so 3.2.1 still applies. See EN 301 549 explained and ETSI's v4.1.1 document.

Equality Act 2010

UK law whose sections 20 and 29 require service providers to make reasonable adjustments in advance for disabled people. It names no technical standard for private businesses. See UK accessibility law and section 29 on legislation.gov.uk.

European Accessibility Act (EAA)

Directive (EU) 2019/882. It has applied since June 28, 2025 to listed consumer products and services, including e-commerce, wherever the provider is established, and service microenterprises are exempt. There is no general transition period for existing websites. See EAA compliance, the EAA by country and the directive on EUR-Lex.

Microenterprise

Under the EAA, an enterprise with fewer than 10 persons and an annual turnover or balance sheet total not exceeding EUR 2 million (Article 3(23)). Microenterprises providing services are exempt (Article 4(5)); product manufacturers of any size are not. See the directive text and EAA compliance.

PSBAR

The Public Sector Bodies Accessibility Regulations 2018 (SI 2018/952), which apply to UK public sector bodies. They require an accessibility statement, and GOV.UK guidance names WCAG 2.2 AA. See UK accessibility law and GOV.UK's guidance.

Section 508

A US law requiring federal agencies' information and communication technology to be accessible. The U.S. Access Board's rule, published January 18, 2017, incorporates WCAG 2.0 Level A and AA, and compliance was required from January 18, 2018. See Section 508 and WCAG and the Access Board's ICT standards.

VPAT

The Voluntary Product Accessibility Template, a free template from ITI for reporting how a product meets accessibility standards. The current version is VPAT 2.5Rev (April 2025), with editions for Section 508, the EU (EN 301 549), WCAG and international use. See VPAT and ACR explained and ITI's VPAT page.

Web Accessibility Directive

Directive (EU) 2016/2102, which covers the websites and mobile apps of public sector bodies. Article 7 requires an accessibility statement with a feedback mechanism. See EU accessibility law and the directive on EUR-Lex.

People and assistive technology

Assistive technology

W3C defines it as "hardware and/or software that acts as a user agent, or along with a mainstream user agent, to provide functionality to meet the requirements of users with disabilities that go beyond those offered by mainstream user agents." Screen readers, screen magnifiers and speech recognition software are examples. See the WCAG 2.2 glossary.

Color vision deficiency (color blindness)

Reduced or absent sensitivity to certain colors, often called color blindness. W3C says these variations in how colors and brightness are perceived can be independent of visual acuity. WCAG 1.4.1 (Level A) requires that color is not the only visual means of conveying information. See use of color, W3C on visual disabilities and the web and the color blindness simulator.

Keyboard accessible

The WAI-ARIA specification defines it as "Accessible to the user using a keyboard or assistive technologies that mimic keyboard input, such as a sip and puff tube." WCAG Guideline 2.1 asks authors to make all functionality available from a keyboard. See the 5-minute keyboard test and WAI-ARIA 1.2.

Screen magnifier

Software that enlarges what is on screen. W3C lists screen magnifiers and other visual reading assistants among assistive technologies used by people with visual, perceptual and physical print disabilities to change text font, size, spacing and color. See the WCAG 2.2 glossary and our color contrast guide.

Screen reader

W3C describes screen readers as assistive technology "used by people who are blind to read textual information through synthesized speech or braille." They work from what your markup exposes, such as headings, labels and alt text. See how people with visual disabilities use the web and our alt text guide.

Situational limitation

A temporary constraint set by surroundings rather than by a disability, such as bright sunlight or a place where audio cannot be heard. W3C counts people with situational limitations among those who benefit from web accessibility. See W3C's introduction to web accessibility and our plain-language guide.

User agent

W3C defines it as "any software that retrieves and presents web content for users." Browsers, media players, plug-ins and assistive technologies are all user agents. See the WCAG 2.2 glossary.

Code and structure

Accessibility tree

The WAI-ARIA specification defines it as a "Tree of accessible objects that represents the structure of the user interface (UI)." Browsers build it from your markup and expose it to assistive technology. See WAI-ARIA 1.2 and semantic HTML.

Accessible name

The WAI-ARIA specification says "The accessible name is the name of a user interface element," and on a button the text "OK" is its accessible name. WCAG calls the same idea a name: "text by which software can identify a component within web content to the user." See WCAG 4.1.2 Name, Role, Value and WAI-ARIA 1.2.

ARIA

WAI-ARIA, the Accessible Rich Internet Applications Suite. W3C says it "defines a way to make web content and web applications more accessible to people with disabilities," especially dynamic content and advanced interface controls. See W3C's WAI-ARIA overview and accessible name.

Heading structure

The outline formed by a page's headings, h1 to h6. Screen reader users can move from heading to heading, for example with the H key in NVDA, so levels should follow the page's outline without skipping. See heading structure, the heading checker and WebAIM's NVDA guide.

Label

W3C defines it as "text or other component with a text alternative that is presented to a user to identify a component within web content." A label is shown to all users, whereas the accessible name may be exposed only to assistive technology. See WCAG 3.3.2 Labels or Instructions and the WCAG 2.2 glossary.

Landmark

The WAI-ARIA specification defines it as "A type of region on a page to which the user may want quick access." Common landmark roles include main, navigation and search. See WAI-ARIA 1.2 and skip link.

Language of the page

The lang attribute on the html element tells browsers and assistive technology which human language a page is in. WCAG 3.1.1 (Level A) requires that the default human language of each web page can be programmatically determined, and WebAIM found a missing document language on 13.5% of home pages in February 2026. See WCAG 3.1.1 and the WebAIM Million.

Link purpose

WCAG 2.4.4 Link Purpose (In Context), Level A, asks that the purpose of each link can be determined from the link text alone or from the link text together with its programmatically determined link context, except where the purpose would be ambiguous to users in general. See WCAG 2.4.4 Link Purpose, the link text checker and W3C's Understanding 2.4.4.

Live region

The WAI-ARIA specification defines live regions as "perceivable regions of a web page that are typically updated as a result of an external event when user focus may be elsewhere." A chat log or a score ticker is a typical example. See status message and WAI-ARIA 1.2.

Meaningful sequence (reading order)

WCAG 1.3.2 (Level A) says that when the order in which content is presented affects its meaning, "a correct reading sequence can be programmatically determined." W3C warns that content which fails it may confuse or disorient users when assistive technology reads it in the wrong order. See WCAG 1.3.2 and W3C's Understanding 1.3.2.

Programmatically determined

W3C defines it as "determined by software from author-supplied data provided in a way that different user agents, including assistive technologies, can extract and present this information to users in different modalities." In practice, information in headings, labels and alt text can be read by software. See the WCAG 2.2 glossary.

Role

W3C defines it as "text or number by which software can identify the function of a component within Web content," for example whether an image functions as a hyperlink, command button or check box. See WCAG 4.1.2 Name, Role, Value and the WCAG 2.2 glossary.

Semantic HTML

Using HTML elements for what they mean, such as headings, lists, buttons, links and tables, instead of styled generic containers, so that browsers and assistive technology can tell what each part is. WCAG 1.3.1 requires that information, structure and relationships conveyed through presentation can be programmatically determined or are available in text. See WCAG 1.3.1 Info and Relationships and W3C's Understanding 1.3.1.

Skip link

A link at the top of a page that jumps to the main content, so keyboard users do not tab through the navigation on every page. W3C documents it as technique G1, "Adding a link at the top of each page that goes directly to the main content area," for success criterion 2.4.1 Bypass Blocks. See technique G1 and the keyboard test.

Status message

W3C defines it as a "change in content that is not a change of context, and that provides information to the user on the success or results of an action, on the waiting state of an application, on the progress of a process, or on the existence of errors." WCAG 4.1.3 (Level AA) requires status messages to be programmatically determined so assistive technology can present them without receiving focus. See WCAG 4.1.3 and the WCAG 2.2 glossary.

Images, media and text alternatives

Alt text

The text in an image's alt attribute, which screen readers read aloud and browsers show when the image does not load. W3C says images "must have text alternatives that describe the information or function represented by them," and good alt text is true of the picture, not just present. See the complete guide to alt text, alt text examples, the alt text checker and W3C's images tutorial.

Audio description

W3C defines it as "narration added to the soundtrack to describe important visual details that cannot be understood from the main soundtrack alone." WCAG 1.2.5 (Level AA) requires it for prerecorded video in synchronized media. See WCAG 1.2.5 and the WCAG 2.2 glossary.

Captions

W3C defines captions as a "synchronized visual and/or text alternative for both speech and non-speech audio information needed to understand the media content." WCAG 1.2.2 (Level A) requires captions for prerecorded audio in synchronized media. See WCAG 1.2.2 and the WCAG 2.2 glossary.

Complex image

A chart, graph, diagram or map that carries more information than a short description can hold. W3C says it needs a two-part text alternative: a short description that identifies the image, and a long description giving the essential information it conveys. See alt text for charts and complex images and W3C's complex images tutorial.

Decorative image

An image that adds no information. W3C says a null (empty) alt text, alt="", should be provided so assistive technologies can ignore it, and WCAG defines pure decoration as "serving only an aesthetic purpose, providing no information, and having no functionality." See decorative images and empty alt text and W3C's decorative images tutorial.

Functional image

An image used to start an action, such as inside a button or link. W3C says its text alternative should convey the action that will be initiated, not describe the picture. See W3C's functional images tutorial and the alt text guide.

Image of text

W3C defines it as "text that has been rendered in a non-text form (e.g., an image) in order to achieve a particular visual effect." Contrast requirements apply to it as they do to live text. See the color contrast guide and the WCAG 2.2 glossary.

Non-text content

W3C defines it as "any content that is not a sequence of characters that can be programmatically determined or where the sequence is not expressing something in human language." Images, charts and icons are the usual examples, and WCAG 1.1.1 (Level A) requires a text alternative that serves the equivalent purpose. See WCAG 1.1.1 Non-text Content and the WCAG 2.2 glossary.

Text alternative

W3C defines it as "Text that is programmatically associated with non-text content or referred to from text that is programmatically associated with non-text content." Alt text is the most common kind. See the alt text guide and the WCAG 2.2 glossary.

Transcript

A text version of the speech and important sounds in audio or video content. W3C lists transcripts and captions among the things people with auditory disabilities rely on. See W3C on auditory disabilities and the web and captions.

Visual design and interaction

Contrast ratio

W3C defines it as "(L1 + 0.05) / (L2 + 0.05), where L1 is the relative luminance of the lighter of the colors, and L2 is the relative luminance of the darker of the colors." Ratios run from 1:1 to 21:1, and Level AA asks for 4.5:1 for normal text and 3:1 for large text. See the color contrast guide, the color contrast checker and the WCAG 2.2 glossary.

CSS pixel

W3C describes it as "the canonical unit of measure for all lengths and measurements in CSS," independent of the density of the display hardware. WCAG sizes such as 320 CSS pixels in Reflow and 24 by 24 CSS pixels in Target Size use it. See the WCAG 2.2 glossary and target size.

Flash

W3C defines it as "a pair of opposing changes in relative luminance that can cause seizures in some people if it is large enough and in the right frequency range." WCAG 2.3.1 (Level A) says pages do not contain anything that flashes more than three times in any one second period, or the flash is below the general flash and red flash thresholds. See WCAG 2.3.1 and the WCAG 2.2 glossary.

Focus

The component on a page that currently receives keyboard input. The Tab key moves focus to the next interactive element, and Shift+Tab moves it back. W3C lists focus as an example of a state of a user interface component (see states in the WCAG 2.2 glossary). See the 5-minute keyboard test and WCAG 2.4.7 Focus Visible.

Focus indicator

W3C defines it as "pixels that are changed to visually indicate when a user interface component is in a focused state." WCAG 2.4.7 (Level AA) requires that any keyboard operable user interface has a mode of operation where the keyboard focus indicator is visible. See WCAG 2.4.7 and the WCAG 2.2 glossary.

Focus order

The sequence in which components receive focus as a person tabs through a page. WCAG 2.4.3 (Level A) requires that, where the sequence affects meaning or operation, focusable components receive focus in an order that "preserves meaning and operability." See WCAG 2.4.3, W3C's Understanding 2.4.3 and the keyboard test.

Keyboard trap

A component that keyboard focus can move into but not out of. WCAG 2.1.2 No Keyboard Trap (Level A) requires that if focus can be moved to a component with a keyboard interface, it can be moved away using only a keyboard interface. Finding a trap means pressing Tab through the page, which is why our methodology lists it as something a static scan cannot see; see also WCAG 2.1.2 and W3C's Understanding 2.1.2.

Large-scale text

W3C defines large scale text as "at least 18 point or 14 point bold or font size that would yield equivalent size for Chinese, Japanese and Korean (CJK) fonts." It is the size when the content is delivered, not text a visitor has zoomed, and it qualifies for the lower 3:1 contrast threshold at Level AA. See the color contrast guide and the WCAG 2.2 glossary.

Non-text contrast

WCAG 1.4.11 (Level AA) asks for a contrast ratio of at least 3:1 against adjacent colors for user interface components and for the parts of graphics needed to understand content. Borders of form fields, icons and chart lines are typical cases. See WCAG 1.4.11 Non-text Contrast, W3C's Understanding 1.4.11 and the color contrast checker.

Reflow

WCAG 1.4.10 (Level AA) requires that content can be presented without loss of information or functionality, and without scrolling in two dimensions, at a width equivalent to 320 CSS pixels for vertically scrolling content, except where two-dimensional layout is required for usage or meaning. W3C notes that 320 CSS pixels is equivalent to a starting viewport width of 1280 CSS pixels at 400% zoom. See WCAG 1.4.10, W3C's Understanding 1.4.10 and CSS pixel.

Target size

WCAG 2.5.8 Target Size (Minimum), Level AA, asks that the size of the target for pointer inputs is at least 24 by 24 CSS pixels, with exceptions for spacing, equivalent controls, inline links, user agent controls and essential cases. W3C defines a target as a "region of the display that will accept a pointer action, such as the interactive area of a user interface component." See WCAG 2.5.8 and the WCAG 2.2 glossary.

Use of color

WCAG 1.4.1 (Level A) says "Color is not used as the only visual means of conveying information, indicating an action, prompting a response, or distinguishing a visual element." An error shown only by a red border fails it, however strong the contrast. See the color contrast guide and W3C's Understanding Use of Color.

Testing, tools and vendors

Accessibility audit

A structured evaluation of a site against a standard such as WCAG, combining automated checks with human testing and ending in a list of issues to fix. See what a website accessibility audit is and what you get and W3C on evaluating web accessibility. GotAlt's deep audit is narrower: it checks whether alt text is true of the image.

Accessibility overlay

Third-party code, typically JavaScript, added to a site to change how the page behaves in the visitor's browser, sometimes with a toolbar of display controls. The FTC's April 2025 order bars accessiBe from claiming its product can make any website compliant with WCAG unless it has competent and reliable evidence (FTC press release). See accessibility overlays and what the FTC found and what to use instead of an overlay.

Automated testing

Checking pages with software that applies rules. W3C says evaluation tools "can not determine accessibility, they can only assist in doing so," and Karl Groves wrote that an automated tool "can definitively test for approximately 25-29% of best practices for WCAG 2.0." See what automated testing can and cannot check, W3C on selecting evaluation tools and our methodology.

False positive

An issue a tool reports that is not a real failure. W3C notes that evaluation tools "can produce false or misleading results," which is one reason a person should check what a tool flags. See W3C on selecting evaluation tools and how to test website accessibility.

Manual testing

Checking a site yourself with a keyboard, zoom, a screen reader and judgment about meaning. W3C says knowledgeable human evaluation is required to determine if a site is accessible (W3C, Evaluating Web Accessibility). See how to test website accessibility and the 5-minute keyboard test.

Monitoring

Re-checking pages on a schedule or after each release so a problem that returns is caught. W3C's planning guidance tells organizations to "continue to review and report on content, processes, and resources." GotAlt's paid plans check monitored pages weekly; see pricing and W3C's planning and managing guidance.

Rule-based scanner

A tool that reads a page's code and applies fixed rules, such as checking that every image has an alt attribute. It can show that alt text is present but not that it is true of the picture, and GotAlt's free scan is rule-based, with 16 checks. See why rule-based scanners miss issues, what we check and W3C on selecting evaluation tools.