When it comes to games...if it looks 3D, it is 3D. Plain and simple. Since true 3D cannot be represented on a 2D surface, what you have is the closest approximation. Super-old FPS games like Wolfenstein 3D and the like were made with raycasters, BSP trees, and other such software tricks because hardware at the time wasn't fast enough to perform the massive number-crunching that "proper" 3D mathematics requires. I have a raycaster-based FPS myself called Two Lords (has nothing to do with the movie...look up "two lords fps" on google).
All that talk about "2.5D shooters" is a load of crap started up by a bunch of nerdy 3D purists who can't wipe their ass in realtime because the toilet paper lags.