Ecology
Also said as Ecosystem, Food chain
What everything in the world eats, and what eats it. A place with real ecology has creatures with reasons to be where they are, doing things unrelated to you.
A monster that stands in a room waiting for the player is a target. A monster that is hunting something else is a part of a world, and it gives the player a whole lot to observe and use.
Say it
“Give the swamp a food chain: frogs eat flies, herons eat frogs, and the big predator hunts herons at dawn. Let the player find that out by watching, and use it as bait.”
Where you’ve seen it
Rain World's creatures search for food and shelter on their own, with their AI running even when they are off screen. In Monster Hunter: World, large monsters fight each other over territory in the "turf wars", and you can use it.
Not to be confused with enemy placement
Enemy placement decides where to put a threat for the player. Ecology decides where a creature would be if the player did not exist.