1) Strict definition: An irrational bias towards members of a specific guild. The bias can be positive (e.g. one guild can prefer the company of its own guild or even another) or it can be negative (e.g. one guild can hate another). To qualify as guildism, the bias must be irrational and/or generalizing. That is, it cannot have a factual basis for preference, and the (mis)deeds of one player can determine how others view his fellow guildmates.
2) Commonly intended definition(in "Guild Wars" computer game): A bias that small guilds have against more succesful guilds.
1. "Man, this guy from guild-x is so dumb. He got his entire raid-party killed today. Guild-x are incompetent."
2. "Ive been watching that guy from guild-y brag about his equipment again, he is not the only one of them who likes to brag. Guild-y is a bunch of showoffs."