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Appendix A Glossary (Normative)


Shortened form of a word, phrase or name, i.e. a general category that includes abbreviations, initialisms and acronyms.


An abbreviation made from the initial letters of a name or phrase that contains several words. Many acronyms can be pronounced as words. Defined differently in different languages. For example, NOAA is an acronym made from the initial letters of the National Oceanic and Atmospheric Administration in the United States.

activity where timing is essential

activity where timing is part of the design of the activity and removal of the time dependency would change the functionality of the content

alternate version

a version that provides all of the same information and functionality and is as up to date as any non-conformant content


A picture created by a spatial arrangement of characters (typically from the 95 printable characters defined by ASCII).

audio description

Audio narration that is added to the soundtrack to explain important details that cannot be understood from the main soundtrack alone. During pauses in dialog, audio descriptions of video provide information about actions, characters, scene changes and on-screen text to people who are blind or visually impaired.

authored unit

Some set of material created as a single entity by an author. Examples include a collection of markup, a style sheet, and a media resource, such as an image or audio clip.

Note: This term was taken verbatim from Glossary of Terms for Device Independence.

background image

Images that appear behind or to the back of the visual field.


Set of technologies assumed to be supported by, and enabled in, user agents in order for Web content to conform to these guidelines.

Note: Some examples of entities that may set baselines that an author may have to follow include the author, a company, a customer and government entities.


turn on and off between .5 and 3 times per second


Synchronized transcripts of dialogue and important sound effects. Captions provide access to multimedia for people who are deaf or hard of hearing.

changes of context

A change of user agent, viewport, or focus; or complete change of content that changes the meaning of the delivery unit.

Note: A change of content is not always a change of context. Small changes in content, such as an expanding outline or dynamic menu, do not change the context.

Computer adapted spatial pattern thresholds

The international definitions for spatial pattern threshold are changing and we will adopt the new standards as they are released. If not available we will remove the L3 SC.


Information in the delivery unit that is used by the user agent to generate perceivable units. This includes the code and markup that define the structure, presentation, and interaction, as well as text, images, and sounds that convey information to the end-user.

delivery unit

A set of material transferred between two cooperating web programs as the response to a single HTTP request. The transfer might, for example, be between an origin server and a user agent.

Note: This term was taken verbatim from Glossary of Terms for Device Independence.


a sudden, unexpected situation or occurrence that requires immediate action to preserve health, safety or property

event handler

A section of code that responds to an action taken by the user (or user agent). On Web pages, events are usually user actions such as moving the mouse, typing, etc. An event handler determines the response to that action. A technology specific event handler only responds to an action by one kind of input device. An abstract event handler is one which can be activated by a variety of input devices.

extended audio descriptions

audio descriptions that are added to an audio/visual presentation by pausing the video so that there is time to add addional description. This technique is only used when the sense of the video would be lost without the additional audio description.


A feature is a specific component of a technology, for example an element in a markup language or a function call in an Application Programming Interface. Typically, a given feature may only be available in specific versions of the technology, and thus may need to be noted explicitly in the required list.

foreign passages or phrases

Foreign passages or phrases are passages or phrases in a language that is different from the language of the surrounding text.


Performs or is able to perform one or more actions in response to user input.

general flash threshold
  • A sequence of flashes or rapidly changing image sequences where both the following occur:

    1. the combined area of flashes occurring concurrently (but not necessarily contiguously) occupies more than one quarter of any 335 x 268 pixel rectangle anywhere on the displayed screen area when the content is viewed at 1024 by 768 pixels and

    2. there are more than three flashes within any one-second period.

    Note: For the general flash threshold, a flash is defined as a pair of opposing changes in brightness of 10% or more of full scale white brightness, where brightness is calculated as .2126*R + .7152*G + .0722B using linearized R, G, and B values. Linearized-X = (X/FS)^2.2 where FS is full scale (usually 255 today). An "opposing change" is an increase followed by a decrease, or a decrease followed by an increase.

idiomatic expressions

words or phrases specific to a region or language that do not mean what the dictionary definitions of the individual words say. For example, the English phrase "he blew his stack" means that someone became very angry.

  1. A message to be sent and received,

  2. A collection of facts or data from which inferences may be drawn,

  3. Knowledge acquired through study, experience, or instruction

information is conveyed by color

perception of the color attributes is essential to understanding a piece of content

information that is conveyed by color

perception of the color attributes is essential to understanding a piece of content


The shortened form of a name or phrase made from the initial letters of words or syllables contained in that name or phrase. Not defined in all languages. SNCF is a French initialism that contains the initial letters of the Societe National des Chemins de Fer, the French national railroad. ESP is an initialism for extrasensory perception.

input error

Any information provided by the user that is not accepted. This includes:

  1. information that is required by the delivery unit but omitted by the user.

  2. information that is provided by the user but that falls outside the required data format or values.


words used in a particular way by people in a particular field. For example, the word StickyKeys is jargon from the field of assistive technology/accessibility.

keyboard interface

An alternate method for connecting a keyboard to the device for the purpose of generating text on devices that do not have a built-in or attached keyboard, or an internal method for generating text. Allowing control via the "keyboard interface" means that the content could be controlled through commands issued from the keyboard or by alternate methods that are capable of generating text as if a keyboard had been used.


Link refers to a hyperlink between the current document and a single destination. (Here, "link" refers to a single "arc" in the XML Linking Language (XLink) Version 1.0 specification.) Only links that are available to be activated by the user need to meet accessibility requirements. This excludes links that are activated automatically or programmatically.

live audio-only

A time-based live presentation that only contains only audio (no video and no interaction).

live video-only

A time-based live presentation that only contains only video (no audio and no interaction).

Lower secondary education level

the two or three year period of education that begins after completion of six years of school and ends nine years after the beginning of primary education.

Note: This definition is adapted from [UNESCO].

luminosity contrast ratio

(L1+.05) / (L2+.05) where L is luminosity and is defined as .2126*R + .7152*G + .0722B using linearized R, G, and B values. Linearized R (for example) = (R/FS)^2.2 where FS is full scale value (255 for 8 bit color channels). L1 is the higher value (of text or background) and L2 is the lower value.


a process or technique for achieving a result


For the purposes of these guidelines, multimedia refers to combined audio and video presentations. It also includes audio-only and video-only presentations that include interaction.

natural languages

languages are those used by humans to communicate, including spoken, written, and signed languages.

navigational features

mechanisms that allow the user to locate and/or move to a different piece of content.

non-text content

Content that is not represented by a Unicode character or sequence of Unicode characters when rendered in a user agent according to the formal specification of the content type. This includes ASCII Art, which is a pattern of characters.

non-text content that conveys information

non-text content content that communicates ideas, data, facts information and is not text.

non-text content that is functional

non-text content that is capable of performing one or more actions in response to user input and is not text.

Note: This includes, but is not limited to, image-based submit buttons, applets, and embedded programmatic objects.

non-text content that is intended to create a specific sensory experience

non-text content that causes a sensory experience that does not primarily convey important information or perform a function.


Required for conformance.

parsed unambiguously

Parsing transforms markup or other code into a data structure, usually a tree, which is suitable for later processing and which captures the implied hierarchy of the input. Parsing unambiguously means that there is only one data structure that can result

perceivable structures

structures in the document that are reflected in the presentation and can be used for orientation and navigation including determining relationships between parts of the content.

perceivable unit

The result of a user agent rendering the contents of a delivery unit. User agents may or may not render all information in a delivery unit. In some cases, a single delivery unit may be rendered as multiple perceivable units. For example, a single html file that is rendered as a set of presentation slides. Most perceivable units contain presentation and the means for interaction. However, for some devices such as printers, a perceivable unit may only contain presentation.


Presentation is the rendering of the content and structure in a form that can be perceived by the user.

Primary education level

the six year time period that begins between the ages of five and seven, possibly without any previous education

Note: This definition is adapted from [UNESCO].

programmatically determined

can be recognized by user agents, including assistive technologies, that support the technologies in the chosen baseline

programmatic user interface component

An interface component created by the author that is in addition to those provided by the user agent. For example, an HTML checkbox would not be a programmatic user interface component because the author is using an interface component supported by the user agent. A checkbox function implemented in script, however, would be a programmatic user interface component because it provides functionality that is not known or supported by user agents and can not be made accessible by user agents even if the user agent complied with UAAG.

real-time events

events that are live and not under the control of the author

red flash threshold
  • A transition to or from a saturated red where both of the following occur:

    1. the combined area of flashes occurring concurrently occupies more than one quarter of any 335 x 268 pixel rectangle anywhere on the displayed screen area when the content is viewed at 1024 by 768 pixels and

    2. there are more than three flashes within any one-second period.

regular expression

A regular expression as defined in XML Schema Part 2: Datatypes, Appendix F.

same functionality

Items are considered to have the same function if the outcome of their use is identical. For instance, a submit "search" button on one delivery unit and a "find" button on another delivery unit may both have a field to enter a term and list topics in the web site related to the term submitted. In this case they would have same functionality but would not be labeled consistently.

same relative order

Each item maintains its position relative to the other items. Items are considered to be in the same relative order even if other items are inserted or removed from the original order. For example, expanding navigation menus may insert an additional level of detail or a secondary navigation section may be inserted into the reading order.

sign language interpretation

providing the translation and meaning of spoken words into specific gestures, hand positions and hand movements.

  1. The way the parts of an authored unit are organized in relation to each other and;

  2. The way a collection of authored units is organized in relation to a delivery unit and;

  3. The way a collection of delivery units is organized

supplemental content

Additional content that illustrates or clarifies default text content, which users may use instead of or in addition to the default text content. For example, there may be supplements in text, graphics, and audio.


Technology means a data format, programming or markup language, protocol or API.


A sequence of characters. Characters are those included in the Unicode/ISO/IEC 106464 repertoire.

text alternative

Programmatically determined text that is used in place of non-text content or text that is used in addition to non-text content and referred to from the programmatically determined text.

unfamiliar content

Content might be unfamiliar if you are using terms specific to a particular community. For example, many of the terms used in this document are specific to the disability community.


Unicode is a universal character set that defines all the characters needed for writing the majority of living languages in use on computers. For more information refer to the Unicode Consortium or to Tutorial: Character sets & encodings in XHTML, HTML and CSS produced by the W3C Internationalization Activity.

URI pattern

A URI pattern is a regular expression identifying a set of resources. A resource belongs to the set if the regular expression matches its URI.

Note: in order to be included in the set, the resource must exist; the regular expression may, and typically will, match URIs that do not refer to any existing resource.

used in an unusual restricted way

words used in such a way that users must know exactly what definition to apply in order to understand the content correctly. For example, the word "representational" means something quite different if it occurs in a discussion of visual art as opposed to a treatise on government, but the appropriate definition can be determined from context. By contrast, the word "text" is used in a very specific way in WCAG 2.0, so a definition is supplied in the glossary.

user agent

Any software that retrieves and renders Web content for users. This may include Web browsers, media players, plug-ins, and other programs — including assistive technologies — that help in retrieving and rendering Web content.

Wisconsin Computer Equivalence Algorithm

The Wisconsin Computer Equivalence Algorithm is a method for applying the United Kingdom's "ITC Guidance Note for Licensees on Flashing Images and Regular Patterns in Television (Revised and re-issued July 2001)" to content displayed on a computer screen, such as Web pages and other computer content. The ITC Guidance Document is based on the assumption that the television screen occupies the central ten degrees of vision. This is not accurate for a screen which is located in front of a person. The Wisconsin Algorithm basically carries out the same analysis as the ITC Guidelines except that is does it on every possible ten degree window for a prototypical computer display.