Wikipedia - Diegesis - Film sound and music

Sound in films is termed diegetic if it is part of the narrative sphere of the film. For instance, if a character in the film is playing a piano, or turns on a CD player, the resulting sound is "diegetic." If, on the other hand, music plays in the background but cannot be heard by the film's characters, it is termed non-diegetic or, more accurately, extra-diegetic. The score of a film is "non-diegetic" sound. Some examples...