2.5D is an optimization trick that fakes the viewer into thinking they are seeing true 3D graphics. A whole scene (or an object in a scene) is made of 2-dimensional graphics that are scaled and drawn in perspective to look like polygonal graphics, but there are no polygons involved. This is done by projecting displacement information onto a single plane. Accordingly, this technique is limited to 2 point perspective, not allowing any Y-axis rotation. The groundbreaking game Doom is built entirely on this concept.
Other effects classified as 2.5D can include parallax scrolling effects or orthographic 3/4 perspective (popularized by many real-time strategy and role-playing games).