Audio Description (AD) is the descriptive narration of key visual elements of live theatre, television, movies, and other media to enhance their enjoyment by consumers who are blind or have low vision. AD is the insertion of audio explanations and descriptions of the settings, characters, and action taking place in such media, when such information about these visual elements is not offered in the regular audio presentation.
(description from The Audio Description Project website, retrieved 1/7/19, http://acb.org/adp/