Counter-Strike (commonly abbreviated to CS) is a tactical first-person shooter video game which originated from a Half-Life modification by Minh "Gooseman" Le and Jess "Cliffe" Cliffe. The game has been expanded into a series since its original release, which currently includes Counter-Strike: Condition Zero, Counter-Strike: Source, Counter-Strike: Anthology and Counter-Strike on Xbox. Counter-Strike pits a team of counter-terrorists against a team of terrorists in a series of rounds. Each round is won by either completing the mission objective or eliminating the opposing force. The latest incarnation of the game, Counter-Strike: Source, is based on the Source engine developed for Half-Life 2.

The game is currently the most played Half-Life modification in terms of players, according to GameSpy^.

Counter-Strike was developed first as a Half-Life modification. Therefore named "Half-Life: Counter-Strike". The original version was a 3rd-party Half-Life modification, but since then it has grown into a commercial mod and later advertised as a separate game in itself. It still uses and runs on the Half-Life game engine and is based on its unchanged structure^.