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.
New games licensed by The Tetris Company call them Tetriminos since 2001, and they were previously called tetraminoes around 1999.
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.
Light blue; shaped like a capital I; four Minos in a straight line. Other names include straight, stick, and long. This is the only tetromino that can clear four lines outside of cascade games.
Yellow; a square shape; four Minos in a 2×2 square. Other names include square and block.
Purple; shaped like a capital T; a row of three Minos with one added above the center.
Green; shaped like a capital S; two stacked horizontal diminos with the top one offset to the right.
Other names include inverse skew and right snake.
Red; shaped like a capital Z; two stacked horizontal diminos with the top one offset to the left. Other names include skew and left snake.
Blue; shaped like a capital J; a row of three Minos with one added above the left side. Other names include gamma, inverse L, or left gun.
Orange; shaped like a capital L; a row of three Minos with one added above the right side. Other names include right gun.
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.
An orientation of each tetromino, with regards to Tetris refers to the different ways that tetromino can exist after rotations.
Normally all the tetrominoes have a total of nineteen orientations, but probably to make things more symmetric, newer games add six orientations: two for S, Z, and I.
Orientations can be described either by the way they point (especially for T, which at least one reviewer has compared to a finger gesture) or by the way the longest flat side faces.