Sound
Diegetic Sound
Sound whose source is visible on the screen or whose source is implied to be present by the action of the film.
In Depth
If the characters can hear it, it's diegetic. This includes dialogue, footsteps, or music coming from a radio in the scene. 'Non-diegetic' sound is added in post-production for the audience only (like the soundtrack).
Example
"A character turns on a car radio and music plays. This is diegetic sound."