One of the all-time classic video games, invented in the mid 1980s.

In its original form, you rotate tetrominoes (groups of 4 connected squares) as they fall into a 10-unit-wide bin; these stick in place as soon as they try to move down but are blocked by another piece, and the next tetromino appears at the top of the screen.

When you complete a row of 10 squares across the screen without any holes, those 10 squares flash and disappear, and the blocks above them drop down a row. If this leaves hanging bits, they remain hanging. If two or more blocks are completed at the same time, they both drop out, and you get a score bonus for doing so. If four rows drop at the same time, which can only be done by standing the long, straight piece on end, this is called a Tetris. The general strategy is to try to avoid covering holes, since this makes the rows containing those holes impossible to drop until the row(s) covering the holes are dropped, although an important strategy is to recognize when you can drop an upper row with a move that seems to cover a hole but doesn't actually because the covering square(s) are immediately dropped.

There are two main variations: Marathon mode, where you just keep playing until you can't fit any more pieces into the bin (or, specifically, until the next piece doesn't fit on the screen without overlapping some other piece), and Goal mode, where you have a set goal, usually a certain number of lines to drop, and after reaching it, the board resets to some initial configuration (often not empty) and you get a new goal. Generally, the Goal mode is used in arcade games where the marathon mode is too short of a game, but both modes are used in computer game versions.

Tetris spawned dozens of imitators and variations. There were games with bigger, smaller, and disconnected pieces, different goals, differently-sized bins, and pieces with various sorts of special effects when they landed; games where two people played against each other, and each line dropped by one player caused some bad effect for the other player (an idea revived by Super Puzzle Fighter 2 Turbo); games where two players played cooperatively in a single, large grid; games based on hexagonal grids and in 3D, etc.