VAC, which stands for Valve Anti-Cheat, is a component of Valve's Steam computer gaming platform that attempts to prevent users from cheating while playing a game. It was first released in 2002 with the game Counter-Strike. By July 2014, it had banned over 2 million Steam accounts for attempting to cheat.
VAC scans the user's hardware and memory, and if it detects any errors that might give the player an advantage, it bans the user at a later date. It does not provide them with the time or date of the recorded infraction. The player is then prohibited from connecting to any VAC-protected servers for the game, and additional restrictions are placed on the user's Steam account.