A cheat that bypasses any restriction codes in most video games. Only noobs use hacks because they find the game un-fair (although deep inside they just naturally suck at the game) or un-fun. So they decide to spice it up a bit by pissing the fuck out of people by always getting headshots and enabling invulnerability.
MegaKilla: Admin dood, that faceboot guy is hacking!
Admin: How can you tell he has hacks?
MegaKilla: He keeps getting headshots, he can't die, and never runs out of ammo!