Pseudo-3D (2.5D)
The umbrella name for every trick that fakes three dimensions in an engine that really only draws in two.
Before cheap 3D hardware existed, games were forced to lie about depth, and they got inventive. Mode 7 scaled and rotated a background layer into a floor; raycasting raised walls column by column in Wolfenstein and Doom; Sega's "Super Scaler" scaled sprites towards the horizon in OutRun; parallax scrolling, billboarding and sprite stacking filled in the rest.
Collectively the tricks are called 2.5D. The term survives even for modern games that render in full 3D but lock movement to a plane — a side-on platformer with real models is as much 2.5D as an old arcade cabinet, just from the other direction. The illusion carried surprisingly far, as long as the camera was not allowed to move freely.