2.5D

learn a technical term:

what is 2.5D?
2.5 D means 2D graphics manipulated to provide 3D effect. the graphics is not actual 3D but 2D, programmed in such a way to make the user think the scene is in 3D

Examples:
increasing the size of an object makes it look like the camera is moving towards the object

placing billboards of texture at different sizes gives a feeling of depth. for e.g placing grass blades at different sizes give a feel of whole grass plains

why 2.5D?
3D graphics computation is expensive in terms of time and memory. hence create 2.5D to give cheap 3D effects, and making the human brain think that the scene is 3D

games using 2.5D:
Outrun (1986), the car moves in a single direction, while palm trees on left and right side of road are placed at different sizes to simulate depth.

Sonic the Hedgehog for the Sega Mega Drive (Sega Genesis in the US) uses parallax scrolling (placing different 2d background images as different layers, to create 3d effects)