A 2.5D game uses either 3D or 2D to simulate the other. In modern day, a 2.5D game normally uses a 3D graphics engine but sticks to a 2D plane (e.g. Trine). More historically it would mean a game that uses 2D images to create the illusion of a third dimension (e.g. Diablo 2).
2.5D ("two-and-a-half-dimensional"), 3/4 perspective and pseudo-3D are terms used to describe either: graphical projections and techniques which cause a series of images or scenes ...