I've seen two mildly differing explanations for this particular version of rendering visuals being called 2.5D, one that it's 2.5D because the geometry looks 3D, but the actors are all sprites, the other is that the geometry looks 3D but it's actually a 2D floorplan with tricks to simulate different heights.

There's also at least one another unrelated "2.5D" use, when the world is 3D, but the player moves on a strictly 2D plane.

Also the fact that the player would just look straight forward the whole time, so when shooting at an enemy at a higher/lower elevation you just needed to overlap him on the X coordinate on screen.

> the geometry looks 3D but it's actually a 2D floorplan with tricks to simulate different heights.

This is the more accurate one. Doom maps are 2D polygons, each polygon has a ceiling height and floor height property.

Eg, a staircase is just rectangles, beside each other, with raised floors.

Aren't all 3D computer games "simulated"?

There must be a better adjective.

Probably. To be clear, what's being stated here is that Doom, Wolfenstein, etc. don't actually render from 3D internal data structures like Quake and later first-person shooters did. The level layout is inherently 2D and some tricks are used to render the environment in a way which looks like a 3D perspective. But it is fundamentally a sort of flat-lander's pseudo-3D.