closed captioning

Short definition from Wiktionary

Noun

1. 1
   The transcription, either verbatim or in edited form, of the dialogue and sometimes sound effects of a program, displayed onscreen and synchronized with the program, most frequently used by the deaf and language learners. Closed captions are encoded invisibly, and displayed by a decoder.
2. 2
   The analogue standard for this transcription, which analogue media like videotapes, laser discs, and cable television use, but also digital forms like satellite television and DVDs.