It means something which looks 3d but is actually 2d under the hood. Doom looked 3d through a combination of techniques but the maps were 2d, Ultima and simcity look 3d because of the isometric perspective but are still just 2d tiles.
Maybe the later ones, but the first SimCity doesn't even look 3D.
It is a highly overloaded term, and it has definitely been applied to doom historically.
It has meant both at different times. Once true 3D games became mainstream, fake 3D games were retroactively re-labeled 2.5D
There even is quite a spectrum between 2D and real 3D. Duke Nukem 3D made heavy use of 2D sprites (so it was less 3D than Quake, and that was good) yet was much more 3D than original Doom.
Those would be classified as "isometric pixel art" -- "2.5D" specifically refers to games like Doom or the Build Engine games that "fake" a vertical dimension from game data that only defines horizontal dimensions.
Doom was called 2.5D because it was not 3D in the sense that you couldn’t stack rooms on top of each other. A given room could have a tall ceiling but there was never a room above or below you.