What is the difference between ‘Diegetic’ and ‘Non-diegetic’ sound?

Diegetic sound is a noise which has a source on-screen. They are noises which have not been edited in, for example dialogue between characters or footsteps.

Another term for diegetic sound is actual sound.

Non-diegetic sound is a noise which does not have a source on-screen, they have been added in. For example music, voiceover, sound effects.

Non-diegetic sounds are often used to add drama to moments that would be silent without it.