A needle drop is when music that wasn’t recorded for a movie is added into a movie – when a pre-existing song plays in a movie or show, either as diegetic sound or non-diegetic sound.

Example: In American Psycho, Patrick Bateman plays ‘Hip to be square by Huey Lewis & The News’ as he is about to murder Paul Allen, the use of an upbeat song juxtaposes his murderous intentions/actions.