To spawn is an online slang term for when something is regenerated or placed on the gaming field. It can refer to both player characters, NPCs and monsters.
Spawning after having been killed is known as respawning. Player characters respawn at a resurrection shrine.
Monsters that spawn because a player moves into a certain area after the instance has been loaded are also said to popup.
The place (or area) where players and mobs commonly spawn is called spawn point.