Immersive sim
Also said as Immersive simulation, Imsim
A world built from systems that follow consistent rules, in which problems have many answers and none is written in by hand. It promises that if your idea should work, it will.
When a door can be picked, burned, blown or bypassed, players start to feel like authors. It is costly, because every new thing must work with all the others, and most players see only one path.
Say it
“Do not script the door. Give wood a burn value and locks a pick difficulty, and let the guard flee fire. If a tester gets through in a way we did not plan, that is a pass.”
Where you’ve seen it
Ultima Underworld (1992) and System Shock (1994) laid the ground. The first level of Deus Ex (2000), on Liberty Island, can be fought through or sneaked through by several routes. In Prey (2017), the foam of the GLOO Cannon hardens into stairs you can climb.
Not to be confused with open world
An open world is about how big the space is. An immersive sim is often a few small dense rooms, and is about how many ways the rules let you cross them.