Valve Anti-Cheat is abbreviated as VAC in Counter-Strike.
VAC is Valve’s anti-cheat system, designed to detect software that gives players an unfair advantage in supported online games. Counter-Strike titles use VAC as part of Valve’s broader effort to protect matchmaking and competitive servers.
A VAC ban is associated with the account detected using prohibited software and can prevent access to protected servers. The system is different from ordinary server administration tools and from community-run moderation. Counter-Strike also uses other measures and reporting systems, so VAC is not the name of every anti-cheat feature in the series.
The abbreviation is sometimes expanded incorrectly as “Valve Anti-Counterstrike,” but the official name is Valve Anti-Cheat. VAC has been used across multiple Valve multiplayer games, not only Counter-Strike.