Drop is the downward motion of tetrominos in Tetris games, and other games that use falling pieces. There are three different types of drops in Tetris, gravity drop, soft drop, and instant drops.
Gravity can be thought of as an automatic drop. It moves a tetromino downward into the playfield at a regular rate, which may change depending on the difficulty level, or progress in the game. Some games move the falling tetromino down the display by a distance of one block every so often; others move it continuously by units smaller than a block. If a tetromino cannot move down any further, it locks into place immediately, or after a short delay.
Gravity is expressed in unit G, where 1G = 1 cell per frame, and 0.1G = 1 cell per 10 frames. Older games cannot move the tetromino down more than one cell per frame (60 cells per second). Newer games, especially those capable of a ghost piece, can do so; play at such speeds requires a lock delay. The term 20G was coined by the developer of Tetris The Grand Master to refer to a speed of 20 cells per frame, which produces the effect of a tetromino instantly falling onto the stack as soon as it appears in the playfield.
Gravity, especially recursive gravity, can also refer to the downward motion of large groups of blocks on the playfield after a line clear.
Many games allow the player to temporarily increase the gravity by holding down a key, most often Down on the directional pad or joystick. Soft dropping a tetromino generally makes it falls at around 20 to 60 blocks per second, as fast as or faster than DAS. Some games will lock a soft-dropped piece as soon as it lands; others, especially Tetris Guideline based, do not lock instantly.
There are two types of instant dropping.
Hard drop is an action that forces the piece land lands and lock instantly. Games that use the Tetris Guideline are required to include hard dropping.
Sonic drop works similarly, but does not lock the piece instantly. Sonic dropping is mainly found in the TGM series, though other games use it as well instead of hard drop. It enables a fast way to fill overhangs using the Zangi-move.