None of the above. It's latency issues. There's such a thing that allows you to increase your own connectivity while at the same time decreasing others. It's measured through Ping tests or some such nonsense.. Anywho, it's basically a guy walking around with a big lag circle around him. He's not affected by the lag, but everyone else is. They think they're shooting him, but he's really 2 or 3 feet forward, or backward, or to the side, than he really is. You just haven't seen him move yet because your connection isn't running as fast as his is. Things like this run rampant through old shooters, like WaW and the first MW, the first Battlefield BC, KZ2, etc. I wouldn't call it a glitch or a bug, as it is something that the player intentionally puts into the game and controls, not something already in the game. I don't think it's a hack either, as hacks generally tend to give infinite health, or shields, or powers, or ammo, or any number of other things, but there is no such thing as giving infinite lag. You can give infinite disconnects by running Botnet DDoS attacks though.

But in all technicality, that's not hacking something; it's bombarding something with so many requests that it can't process and run. Maybe it can be considered an exploit? Hell, who even cares about the terminology?