Why is Duke Nukem 3D not really 3D?

Question

Game development

I found info on the net that Duke Nukem 3D is actually not 3D, but "pseudo-three-dimensional".

Alas, I could not find an explanation of this in the search engine, except that, supposedly, you cannot be above another object. However, in the game you can fly, jump, even on the heads of monsters and other objects, such as tables, chairs, there are even elevators.

How to understand if a game is 3D or pseudo-3D? Why does Duke feel like a completely 3D shooter? Why is this not true 3D?

Answer 1

Quote from Wikipedia:

However, the Build Engine was still a 2.5-dimensional engine, and besides, when the player looked up, the objects were greatly distorted: there was a distorted perspective effect

Answer 2

3d is when 3-dimensional models are honestly calculated, 2.5d is 2-dimensional models that are drawn as 3-dimensional ie. they have perspective/depth, for example enemies - consist of 3-6 sprites - front, back, side and top view - like a target in a shooting range