Diegetic sound is sound whose source exists within the world of the film — heard by the characters, not just the audience.
Diegetic sound — from the Greek 'diegesis', the world of the story — refers to any sound whose source belongs inside the film's narrative reality: dialogue between characters, footsteps, a radio playing in a kitchen, a band onstage. Non-diegetic sound, by contrast, exists only for the audience: the orchestral score, voice-over narration, sound effects added for mood. The distinction was theorised by film scholars in the 1970s and has become essential vocabulary because directors play with the boundary deliberately. In Robert Altman's 'Nashville' (1975) the score and the source music blur until you cannot tell which is which. In David Lynch's 'Mulholland Drive' (2001), a song performed onstage continues after the singer collapses — sound revealing that the world we are watching is itself a fiction. Listening for the diegetic line is one of the surest ways to understand how a film constructs its reality.