From http://lists.w3.org/Archives/Public/public-comments-wcag20/2006May/0119.html
audio description
narration added to the soundtrack to describe important visual details that cannot be understood from the main soundtrack alone. [...] Audio descriptions of video provide information about actions, characters, scene changes, and on-screen text. [...] In standard audio description, narration is added during existing pauses in dialogue.
1. Audio description provides ""information about actions, characters, scene changes, and on-screen text"" among other things. Nobody has produced an exhaustive list, and maybe we do or do not need such a thing, but it isn't limited to those items.
2. Audio description is typically ""added during existing pauses in dialogue."" There are quite a few occasions in which it is necessary to describe over dialogue.
3. ""Audio description"" is generally written as a mass noun, like ""captioning."" Hence ""audio description of video provides.""
WCAG 2.0 defines audio description (AD) as an alternative that is inserted during natural occurring pauses. We cannot require AD to occur over dialogue, even if there is not enough space to fully describe the content, because putting AD over dialogue would make that dialogue inaccessible. Level AAA requires extended audio description which is usually how this should be handled.
Although AD is not yet standardized as a Mass noun (http://ncam.wgbh.org/richmedia/tutorials/audiodesc.html), there is momentum in the industry to make it a mass noun and therefore we have changed it according to your recommendation.
We have also revised the first note in the definition of audio description to read, "Audio description of video provides information about actions, characters, scene changes, on-screen text, and other visual content."