How can a game be 2.5D?

I've noticed some games be described as 2.5D. How can a game be 2.5D? Dimensions are discrete entities, aren't they?

-
True 2.5D. I believe the video is fake, but it is fairly awesome. – Corey Jul 15 '10 at 5:04
Tangentially related, but 2.5D is commonly used to refer to CNC routers as well, where they can move in X and Y freely, but they only have 2 Z positions, down (cutting) or up (not) – Nick T May 4 '11 at 14:52
"Dimensions are discrete entities, aren't they?" - Not if you talk about fractal dimensions - en.wikipedia.org/wiki/Fractal_dimension – Atav32 Jun 6 '12 at 18:02
@Corey I'm not sure, he lets you download an exe, but I don't have a windows machine right here to test it. Still, it's incredibly awesome, and also predates Portal 2!!! – o0'. Jul 25 '15 at 20:41

2.5D usually refers to "gameplay in an otherwise three-dimensional video game that is restricted to a two-dimensional plane."

It can also refer to the viewpoint the player has. Such as an isometric-type view on a 3D world.

Wiki - 2.5D

-
2D games with sprites rendered at a isometric angle are also considered 2.5D so it's not limited to 3D games. – Dave Nov 3 '15 at 6:48

Orthogonal-view games like Diablo are "2.5D":

-
Thanks for the illustration. Though I would have appreciated some freehand circles. – mechko Jul 15 '10 at 3:07
Fact: Diablo is still fun. – Chris Jul 15 '10 at 3:13
@Mechko: I dunno, that big blue glowy thing in the middle of the shot looks pretty freehand to me.... – Pops Jul 15 '10 at 21:27
@Chris is it still? – ardaozkal Nov 19 '15 at 16:29
@ardaozkal been thinkin bout what Wirt's got in stock all day, yo. – Chris Nov 19 '15 at 17:56

The original 2.5D game was DOOM. They called it 2.5D because it did feature all three dimensions (length, width, and height), and allowed map makers to create maps that made prominent use of the third dimension, but it still wasn't true 3D in two important senses.

First, sectors (i.e., rooms) could not be placed on top of other sectors, so the overall layout remained a strictly 2D diagram that allowed different sector heights. And second, the DOOM renderer was not a true 3D renderer; it was highly optimized for 2D plus heights but could not have rendered true, arbitrary, 3D geometry unlike Quake.

-

2.5D can refer to a few things:

• A game which has fundamentally two-dimensional gameplay but features a 3D presentation (see Shadow Complex).
• A game which has fundamentally three-dimensional gameplay but features a 2D or partially 2D presentation (see Duke Nukem 3D).

There's also a very uncommon, loose definition:

• A game which has not-quite two-dimensional or three-dimensional gameplay and/or presentation. For example, I believe Starcraft has an internal concept of "high ground" even though it has a 2D presentation. Or, something like Little Big Planet which is really just three 2D planes stacked together.
-
I'd call LBP a 2.75d game because not only is it presented in 3d, it also has a limited 3d gameplay aspect to it. – RCIX Jul 15 '10 at 6:56
TrackMania (free racing game) has a 3D option that'll appear in 3D with the red/blue glasses available for watching 3D movies. – Bernhard Hofmann Jul 15 '10 at 10:03
@Bernhard - that's a different sort of 3D, sometimes called stereoscopy. In this case the OP is talking about games that have the illusion of depth but don't actually have a 3D game world representation. – ChrisF Jul 15 '10 at 21:48

2.5D refers to any time that there's really only 2D stuff going on, but they "fake" a third dimension. Orthogonal view games like Diablo fake it because they don't have to do full 3D rendering, but they can give the appearance of 3D.

-