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Glossary definitions are normative. Their notes are informative.

captions

synchronized visual and/or text alternative for both speech and non-speech audio information needed to understand the media content

Informative notes (6)

Captions are similar to dialogue-only subtitles except captions convey not only the content of spoken dialogue, but also equivalents for non-dialogue audio information needed to understand the program content, including sound effects, music, laughter, speaker identification and location.

Closed Captions are equivalents that can be turned on and off with some players.

Open Captions are any captions that cannot be turned off. For example, if the captions are visual equivalent images of text embedded in video.

Captions should not obscure or obstruct relevant information in the video.

In some countries, captions are called subtitles.

Audio descriptions can be, but do not need to be, captioned since they are descriptions of information that is already presented visually.

image of text

text that has been rendered in a non-text form (e.g., an image) in order to achieve a particular visual effect

Informative notes (1)

This does not include text that is part of a picture that contains significant other visual content.

text

sequence of characters that can be programmatically determined, where the sequence is expressing something in human language

assistive technology

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

Informative notes (3)

Functionality provided by assistive technology includes alternative presentations (e.g., as synthesized speech or magnified content), alternative input methods (e.g., voice), additional navigation or orientation mechanisms, and content transformations (e.g., to make tables more accessible).

Assistive technologies often communicate data and messages with mainstream user agents by using and monitoring APIs.

The distinction between mainstream user agents and assistive technologies is not absolute. Many mainstream user agents provide some features to assist individuals with disabilities. The basic difference is that mainstream user agents target broad and diverse audiences that usually include people with and without disabilities. Assistive technologies target narrowly defined populations of users with specific disabilities. The assistance provided by an assistive technology is more specific and appropriate to the needs of its target users. The mainstream user agent may provide important functionality to assistive technologies like retrieving web content from program objects or parsing markup into identifiable bundles.