According to Wikipedia…

A tetromino, also spelled tetramino or tetrimino, is a geometric shape composed of four squares, connected orthogonally. This is a particular type of polyomino, like dominoes and pentominoes are. The corresponding polycube, called a tetracube, is a geometric shape composed of four cubes connected orthogonally.

Sound familiar? It should. Here are a couple of examples:

I find the etymology here particularly interesting…

The word polyomino and the names of the various orders of polyomino are all back-formations from the word domino, a common game piece consisting of two squares, with the do- beginning fancifully analyzed as a version of the prefix di- meaning “two.” The origin of the word domino for the game piece is not certain.

Most of the number prefixes are Greek. Polyominoes of order 9 more often take the Latin prefix nona- (nonomino) than the Greek prefix ennea- (enneomino).