@tkn
@CodingItWrong That wiki page covers most of the detail, but to offer a more pithy explanation: it's all semantics, as all 3D games are ultimately rendered as a 2D image, but when people say "real" or "true" 3D they usually mean polygonal 3D, where the maps and all objects are stored and manipulated as 3D coordinates -- including the player, enemies, and camera.

Doom looks 3D thanks to its use of billboarding (for sprites) plus ray casting and binary space partitioning (for walls and floors), but you could render it as a top-down maze and it's mechanically the same game. Marathon uses shearing (sliding and scaling on the y axis) to create an optical illusion that you're looking up and down but is also using tricks to present 2D geometry and logic as though it's 3D.