Valve Anti-Cheat. Basically, it's a program that is like a virus scanner only for hacks, if they detect hacks on server joins and whatnot, you will be banned within a few days, hours, or weeks.

Players who have been banned always claim they didn't cheat, or their brother or friend put the hacks on, basically trying to get it out. There have only been less than 10 people actually unbanned from VAC before. Anyone who is banned will only be banned from VAC secure servers, but can still play on unsecured ones. Every game that uses VAC will be inaccessible (you won't be able to play on VAC secure servers).

It's often criticized as a way to make money from Valve to get a new account and games. Accounts that have been banned payed up to thousands on games before, but over 1 hack they loose all.

Many times VAC isn't updated to the most recent hacks, private hacks still exist, and are harder to detect.

VAC has certainly busted many cheaters, but some still get by.
Noob: OMG! Me is VAC banned! WTF! vavle u just want my monies! I didtn hack!

Me: God shut up. We all know you hacked, headshots 24/7 isn't easy, but you managed to do it. Go away.
by Da Milkman June 26, 2009
Valve Anti-Cheat

Technology used in Steam games like Half-Life, Counter-Strike etc. to stop cheaters and hackers. Anyone who cheats will immediately be banned from all servers running VAC.
<Player 2> gtfo ****ing h4x0r
by chstrike April 3, 2004
Some think it's cheesey dance music, others think it's crappy power noise, some think it's the best thing since sliced bread.
If you haven't heard more than at least 3 Velvet Acid Christ albums, then you really aren't in a position to judge.
There are many different styles found here, including industrial, goth, electro, noise, psy-trance and "strange techno", if you are a fan of anyone like The Cure, The Sisters of Mercy, Nine Inch Nails, Skinny Puppy or just want to find something different, Velvet Acid Christ is not to be missed.
VAC are frigging great!
by hinotoxin September 30, 2005
Acronym for EBM artist Velvet Acid Christ.
Dude, this VAC crap just sounds like power noise. How the hell can you dance to this?
by Siegfried Zaga May 21, 2005
To be banned on a game server run by valve that is VAC secured
e.g. Tf2, CSGO, Half life, etc.
Stupid Guy: Oh man i got vacced yesterday
Me: How?
Stupid Guy: I was trying to hack tf2
Me: Idiot
by BOBYFUR242 February 26, 2017
Michael bought a Dirt Devil vac from Wal-Mart.
by hoyclan August 11, 2016
Abbriviation for Vacation

Usually seen in a working environment in the form of Email. It is commonly used to notify a group of people if someone will be away from work.
To: John Doe
From: Jane Doe
Subject: vac

I will be on vaction Tues-Thurs returning to work Friday.
by Zachry Diehl September 15, 2006