From Hard Drop - Tetris Wiki
A piece in Tetris and similar games is a geometric shape consisting of a set of blocks that is moved as a unit. A block is the part of a piece that fills one unit of the playfield. In some games, all blocks that make up a piece are colored the same; in others, especially falling piece games other than Tetris and its clones, they are colored differently.
A polyomino is a piece made of two or more square blocks, where all blocks are connected through full coincident edges (as if squares on graph paper).
- A free polyomino is one that may be translated (moved about), rotated, or reflected.
- A one-sided polyomino is one that may be translated or rotated.
- A fixed polyomino can only be translated.
Polyominoes come in several sizes:
- 2 blocks make a domino: featured in Dr. Mario and Puyo Pop; seen in Tetris Blast.
- 3 blocks make a tromino (or trimino): featured in Columns (fixed I only) and Pac-Attack (L only); seen in Tetris Blast (I, L) and Puyo Pop Fever (L).
- 4 blocks make a tetromino (or tetramino or tetrimino): featured in Tetris and its clones. Lumines uses only O tetrominoes.
- 5 blocks make a pentomino: featured in Daedalian Opus; seen in Tetris 2 + BomBliss and Magical Tetris Challenge.
Polyominoes are often named by a Latin letter that resembles the shape of the polyomino. For instance, an "I" has all its blocks in a straight line, and a "T" consists of an "I" tromino with one or more blocks in a straight line connected to one side of the center of the "I".
 Hinged polyominoes
| Hinged tetrominoes|
in Tetris 2
A hinged polyomino or polyplet is similar to a polyomino but also allows connections at a point. In games that use hinged polyominoes, a piece may break apart at the diagonal connections when it lands, and the fragments may or may not be controllable afterward.
 Other polyforms
Hextris and Bikaka are variants of Tetris that use 4-cell polyhexes, or pieces created from regular hexagons. Bubble Bath Babes uses polyhexes that may be reflected around the X or Y axis but not rotated.