Cheating or hacking programs in online multiplayer games such as Counter-Strike. The cheater often has an aimbot, can see players through walls, uses neon colored player models, and has decreased recoil to name a few advantages.
That guy just got three headshots in a row! He must be using h4x!
A term commonly used towards someone who is playing amazing, or doing things which may look like they are cheating, when in fact, in most cases, they are not. Also used a complement to good players labelling their skills as good enough to be considered cheating.