In MMORPG terminology, an NPC, an acronym for Non-player Character, are characters that don't join the players' parties and are not controlled by real players, being created only to fulfill a certain role, like starting an Event or being a shopkeeper. The System Administrators of The World frequently disguise themselves as NPCs in order to interact with the players without being noticed.
NPCs should not be confused with Vagrant AIs, as Vagrant AIs can think and act for themselves, while NPCs only act in the way they were programmed to.