What is the difference between 'diegetic' and 'non-diegetic' sound?

'Diegetic sound' refers to any sound that comes from the world shown on screen, that is the source of the sound in part of the on screen environment. Some examples of diegetic sound include: a song playing from a radio on screen, dialogue spoken by characters (note: a voiceover, where the speaker is NOT seen is non-diegetic!), or the sound of a car passing.

'Non-diegetic sound' is all sound that does NOT originate from the world of the screen. For example, all score/orchestral/incidental music is non-diegetic.