The widely accepted definition is: 2D gameplay, 3D art.mclem said:To be fair, it's a bit of a slang term that I've heard having two or three different definitions. I first heard the term to represent Doom, where the maps were heavily-disguised 2D rather than true 3D. That's certainly not consistent with "2D gameplay, polygonal graphics".
Only very rarely do people use it in any other fashion these days.