Nowadays, any game that doesn't use polygonal graphics is considered 2D. But, I've seen a lot of old articles that casually mention games like Zaxxon as being 3D (and I'm not talking about Zaxxon 3D, I mean the original). I vaguely remember one article from the early 90s that talked about "different kinds of 3D" and listed them: polygonal, vector, scaling sprites, isometric. I suppose 3D vector games (like Star Wars) never lost their 3D status, but the latter two are definitely not considered 3D anymore. For a while in the early to mid 90s, I remember the phrase "isometric 3D" being used to describe games like LandStalker and Super Mario RPG.
So, what qualifies a game as being "true" 3D? There's a lot of grey areas. You could say a game is 3D if it allows for three axis of movement (therefore having three dimensions), but then that would include games like Double Dragon, which are pretty clearly not 3D (or are they?). If the qualifier is polygonal graphics, then that raises the question of games that partly use polygonal graphics, like Resident Evil or Grandia.
I was talking to someone on IRC and he mentioned LandStalker as a 2D game. I didn't think LandStalker was really 2D, and I asked him if he also considered Dark Savior to be 2D. He said he wasn't sure. It's something I've never really thought about.
Anyway, I'm rambling. Thoughts?