"Gameplay restricted to a 2D plane taking place in a 3D environment."
as max said. This is way how game developers understand 2.5D term in games (2.5D in CG graphic programs, like ZBrush have different meaning), and there isn't 2.2D, 2.3D or 2.1243D. Levels of depth have nothing common with 2.5D game definition. It's still 2.5D no matter how it is exactly build. If "Gameplay
restricted to a 2D plane taking place in a 3D environment", then game is 2.5D
Not sure if that what i just wrote is clear 