The drop is the point in a track where a switch of rhythm

Rhythm

Rhythm may be generally defined as a "movement marked by the regulated succession of strong and weak elements, or of opposite or different conditions." This general meaning of regular recurrence or pattern in time may be applied to a wide variety of cyclical natural phenomena having a periodicity or...

 or bass line occurs and usually follows a recognizable build section and break

Break (music)

In popular music, a break is an instrumental or percussion section or interlude during a song derived from or related to stop-time – being a "break" from the main parts of the song or piece....

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In Hip-Hop and electronic music, the reintroduction of the full bass line and drums is known as the drop.

In Dubstep

Dubstep

Dubstep is a genre of electronic dance music that originated in south London, England. Its overall sound has been described as "tightly coiled productions with overwhelming bass lines and reverberant drum patterns, clipped samples, and occasional vocals"....

, the drop involves a full bass line and commonly a "wobble" bass or "vowel" bass accompanied by a strong shuffling beat.

In Metalcore

Metalcore

Metalcore is a subgenre of heavy metal combining various elements of extreme metal and hardcore punk. The name is a portmanteau of the names of the two genres. The term took on its current meaning in the mid-1990s, describing bands such as Earth Crisis, Deadguy and Integrity...

 sub-genres, bass drops are often utilized under the first chord of a breakdown, to emphasize the breakdown and give it a pronounced presence.

Double drop

In drum and bass

Drum and bass

Drum and bass is a type of electronic music which emerged in the late 1980s. The genre is characterized by fast breakbeats , with heavy bass and sub-bass lines...

, skilled DJs sometimes perform what is called the "double drop": beat matching two tracks in a way that the drop, and hence the respective climaxes, occur at the same time.