Endless opposition

Generally this is either due to technical limitations (not tracking who's been defeated), to force players to progress, or to provide an endless source of supplies in case they drop loot or can be scavenged for items.

This does not include respawning that occurs only when a game is loaded, such as in Diablo games where you can kill everyone as long as you don't re-load your game. "Screens" being repopulated on re-entry does not count either unless it is mandatory to return to them.

Contrast: finite opposition

Related:
* NPC re-spawning
* Random encounters

Contrast:
* Elimination - it's fairly impossible for the game to be about elimination if you can never accomplish it.