Tetromino

A tetromino, is a polyomino made of four square blocks. The seven one-sided tetrominoes are I, O, T, S, Z, J, and L.

The spelling "tetromino" is standard among mathematicians. The Tetris Company had preferred tetramino around 1999 and has used tetrimino since 2001.

The basic tetrominoes
Each tetromino has a letter name and several alternative names.

In most games other than Tetris for NES, Tetris & Dr. Mario, or black-and-white systems, each tetromino has a different color. However, before about 2001, these colors varied from product to product, making it difficult for people who play different games to discuss "the blue piece". Since then, the color schemes associated with SRS and TGM Rotation have dominated new games, and "the blue piece" is always J.

I
Other names include straight, stick, and long. This is the only tetromino that can clear four lines outside of cascade games.

O
Other names include square and block.

S
Other names include inverse skew and right snake.

Z
Other names include skew and left snake.

J
Other names include gamma, inverse L, or left gun.

L
Other names include right gun.

Multimino
In The Next Tetris, Sticky mode of Tetris Worlds and Tetris Mania, and Jewelry Master, some tetrominoes are multiminoes, that is, the tetromino itself is divided into visible components. Though the tetromino is moved and rotated as a unit, once it locks, the components separate by colors and fall.