Valve Anti-Cheat (often abbreviated as VAC) is an anti-cheat software that is utilized in Valve's Steam platform for eligible online games. It was released in 2002 with Counter-Strike as a way for Valve to solve the problem of its major cheating base online. Since then, developers are able to apply this software to their own games via Steamworks, Valve's API.
The software can detect cheating in online games through an advanced algorithm, and can kick or ban players from certain games as a penalty. The amount of bans a player has may be seen on their profile, under the name "VAC Bans" in red text to the right of the screen, as well as how many days since their last one. Players that are under the effect of a VAC ban may be subject to additional punishment.