NPCs, monsters and any other enemies (and possibly allies) are (re-)spawned/repopulated occasionally, either by time trigger or location triggers such as distance or level change.
Alternate names: Monster spawning, NPC respawning
The first video game about NPC spawning was released in 1981.
* Random encounters
* Monster generator - in case the spawning can be stopped by destroying an object.
* Pop-up mooks - respawning process may appear like creatures are popping up out of nowhere, but players normally don't see it (unless they have a habit of running back and forth rapidly).
See also: NPC refills where areas are repopulated by unrelated things
Although unrealistic in most cases, this provides some extra entertainment for players when they backtrack/revisit locations they have been to earlier. No longer finding them devoid of life as they left them. Usually coupled with corpses and dropped items disappearing at similar pace.
There are two common approaches to produce NPC (re-)spawning.
* Literally respawn the creatures at their "start" locations, some have the decency of adding a teleport-in effect or similar.
* Spawn new replacements that arrive from "outside", whether they're exactly the same as the old is irrelevant.
* NOT for simulated breeding
* NOT for devices that spawn creatures that can be stopped indefinitely
The spawning process is usually limited to replacements, but can occur without such limit.