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How can a game be 2.5D?

I have been told that when the game is basically a side-scrolling game with 3D elements in the background/environment, it becomes a 2.5D game. Is there any correct explanation for this term?

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Agreed, but when I typed in the question, the possible duplicate thing never showed up. I tried 2.5D first and changed it to the wordy version cuz of the minimum length requirement – Lelouch Lamperouge Dec 29 '11 at 9:16

marked as duplicate by Mr Smooth, Oak♦, Dave DuPlantis, LessPop_MoreFizz, BlueRaja - Danny Pflughoeft Dec 29 '11 at 23:11

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Games are said to be in 2 dimensions, when you can only move in the X and Y axis (ie. left, right, up and down or similar).

3 dimensional games let you move in more more axis (usually the Z axis).

A 2.5 dimensional game is a game that works much like a 2 dimensional game, but usually allows you some limited control of the third (Z) axis. An example of this would be Little Big Planet, where you usually just move in the X and Y axis, but can step "into" the Z axis in a limited fashion (I think there are like 3 "planes", front, middle, back). There are also some fighting games that allow this.

This is not to be confused with games that are done with 2d or 3d graphics.

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The meaning of the term has actually changed over time. It used to mean games that used 2d graphics to appear 3d, such as Doom. But now, as described above, refers to games that use a 3d engine to create a game which is only played in 2 dimensions. – Kurley Dec 29 '11 at 9:27
@Kurley: I think a better example is isometric games (like old RPGs, or the original Starcraft) where there appears to be a third dimension (height), even though the game is entirely 2D tile-based; or old 2D racing games which appeared to have depth by making objects larger as they got closer. – BlueRaja - Danny Pflughoeft Dec 29 '11 at 23:25
@BlueRaja-DannyPflughoeft: Not really, those were already given the name Isometric. 2.5D came about to distinguish proper 3D like quake and unreal etc, from 3D-esque (but sprite based) 3d like Wolfenstein, Doom, Blood, DukeNukem3D, etc. – Kurley Dec 30 '11 at 9:37
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One correct interpretation is in Holger's answer. In more technical articles however, you may encounter a substantially different usage of this term.

More specifically, it indicates that the third dimension is represented in some restricted way.
When you use a 2D representation for objects, you specify their points in 2D space, i.e. using 2 coordinates. [Think (x, y).] Similarly, for 3D objects you use three coordinates (x, y, z).
But in some scenarios, you don't need the full third dimension. In many games, for example real-time strategies, terrain is represented using 2D coordinates and height, specifying its elevation at each point. The plus side is it simplifies the math involved, downside being that since for each point on the map you can only specify one height, you can't create overlapping structures, caves, etc.

Random image from the web Since there are no overhangs in the picture above, it could be represented in 2.5D.

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