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.
Players who have been VAC banned may not play the game they received the VAC ban from. If a player was VAC banned from a game or mod which uses the Source or Gold Source engine, they will be banned from all games using said engine. Players are still able to play single-player games, LAN games, and multiplayer games on servers that are not VAC secured. Players under the effects of a VAC ban may also not upload new content to game hubs or vote on Workshop content. Finally, players will no longer be able to gift the game they have been VAC banned from to another player.