What is the definition of diegetic and non diegetic sound?

Diegetic sound is sound that comes from the setting of the film. Non-diegetic sound is sound that comes from our world, such as the soundtrack or scoring. Diegetic sound could include the voices of characters, utensils clattering in the background, or music coming from a piano being played on-screen.

What is a needle drop in film?

Needle drop (movie scoring), the use of an existing recording rather than an original score in a film.