The standard Tetris mechanic that lets a player reserve one piece for later use is hold.
With hold, a player stores the currently falling tetromino in a separate holding area and swaps it with the active piece. The reserved piece can then be brought back later, usually once per falling piece. This gives players a way to save a useful shape, such as an I tetromino, or escape a difficult placement.
The feature became associated with modern guideline Tetris through The New Tetris, released in 1999. It was later incorporated into the standard rules used by many contemporary Tetris games, including competitive and online versions.
Hold is different from a hard drop or soft drop, which change how quickly a piece descends. It is also distinct from a T-spin, an advanced maneuver involving rotation and placement of a T tetromino. Not every early Tetris version included hold, so it is a standardized modern feature rather than part of the original 1980s design.