A drop in popular music, especially electronic music styles, is a point in a music track where a switch of rhythm or bass line occurs and usually follows a recognizable build section and break.
The term "drop" comes from the composer or producer "dropping in" the primary rhythmic and foundational elements previously hinted at into the mix more or less at once. Related terms, typically describing certain types of drops, include "beat-up" (so named because it is a point where the producer brings up the foundational kick drum beat after having faded it down during a break or buildup) and "climax" (typically describing a single particularly striking drop heard late in the track).
Many genres of EDM can have more than one drop during a track, especially if the song is built on a "dance-pop" verse/chorus with vocals; a drop of some kind is typically heard somewhere during each chorus as the high point of that verse/chorus cycle. Most genres, however, tend to emphasize a single drop as the beginning of the high point, or climax, of the entire track; in vocal subgenres this is typically the last repetition of the chorus, while in nonvocal genres it typically occurs in the last quarter of the track.