Gaming is a term used when a gamer gains what is deemed to be an unfair advantage (either by the game designers and/or other players). This generally applies to MMORPGs and includes (but is not limited to):
Opportunistic - Exploiting bugs/flaws in the game.
Physical - Creating extra accounts (commonly referred to as mules or alts). This is the most common form of gaming.
Social - Taking advantage of newbies to achieve your ends.
Technical - Entering cheats (or otherwise modifying the program code/communications).
In an environment where this behaviour is acceptable, the gamer generally prefers to be called a "powergamer" or even "metagamer"