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.
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.