Diegetic Sound is sound whose source exists within the world of the film's story.
Diegetic sound originates from a source within the film's fictional world — dialogue between characters, a radio playing in the background, footsteps, a gunshot — meaning that, in principle, the characters themselves could hear it. Non-diegetic sound, by contrast, is added for the audience alone: a swelling orchestral score, a narrator's voiceover, or ominous stingers that no character on screen registers. Filmmakers exploit the boundary between the two constantly for effect — a piece of music that seems non-diegetic can reveal itself to be diegetic when the camera pans to reveal a character switching on a radio, or a director may deliberately blur the line to disorient the audience. The distinction, borrowed from narrative theory's concept of the 'diegesis' (the story world itself), gives filmmakers and critics a precise vocabulary for something viewers intuit long before they can name it.