Audio Description (AD) is descriptive audio narration of relevant visual elements of a video not represented in the original audio track for the benefit of people who are blind or low vision.
There are two main types of AD for videos:
Standard AD is written and recorded so that the audio descriptions fit within the already existing gaps between dialogue and important audio elements. Example.
Extended AD, however, is audio description added to a video through pausing. These video pauses allows time to add additional description of what is happening on screen. This technique is used when the sense of the video would be lost without the additional audio description and when the existing gaps are too short. Completed Extended AD videos are therefore longer in duration than the original video. Example.