"Two and a half-D" is an optimization trick for RT3D that fakes the [[Viewer1|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. Billboards are 2.5D, but voxels can be either 2.5D or 3D.
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).