What is the definition of diegetic/non-diegetic sound in film?

Diegetic elements are part of the fictional world (“part of the story”), as opposed to non-diegetic elements which are stylistic elements of how the narrator tells the story (“part of the storytelling”)

(source – Wikipedia)

What is a needle drop in film?

the use of an existing recording rather than an original score in a film.

(source – Wikipedia)