Diegetic - spatio temporal universe referred to by the primary narration.

In sound may include sfx, ambiance, dialogue, anything that functions to enhance the verisimilitude of the image.  

May be off-screen or asynchronous.

Non-diegetic audio has no clear source in the story space.  Music scoring (also known as score or “pit music”), narration, etc.  

Diegetic music is known as source music or “screen music”.