(1) Slow alteration of the level of a signal, usually using a potentiometer. See fade-in/fade-out.
(2) Of a piece of music, usually commercial music, the repeated section at the end of the song which is subjected to a gradual fade-out. See also outro.
(3) Short for fade-in/fade-out. Optical effects in which a scene is printed with exposure increasing or decreasing to blackness for fade-in and fade-out, respectively.