A term used in Massively Multiplayer Online Role-Playing Games (MMORPGs) to describe a player who obtains items en masse (i.e. farming) and in-game gold, to sell to other players for real-world money.
Gold-farmers are usually also ninjas (people who take all the loot from a team-kill), which not only results in poor team-play, but also causes major problems with the in-game economy. For example, in World of Warcraft, some servers experience extremely low prices on the official auction house for certain items as they are being farmed by gold-farmers. This means that genuine players trying to sell those items cannot do so.
Gold farming is usually illegal within the game, and the creators of such MMORPGs will typically kick gold-farmers from the system when/if they are discovered.