2.5D is a term today mainly used to describe games where the graphics are polygonal, but the gameplay takes place on a two-dimensional plane ( Duke Nukem: Manhattan Project, Kirby 64: The Crystal Shards, Shadow Complex). Once a technique used as something of a novelty, modern game development has shown a greater move toward polygonal graphics for most genres regardless of gameplay, and therefore a decided increase in the amount of 2.5D games being released.
Today, the term "2.5D" can normally be associated with games that have polygonal graphics in a 3D environment, and a fixed-perspective camera. The moment-to-moment gameplay will primarily take place on a 2D plane. Even if the mechanics of the engine allow for variations and scripted sequences, the player will seldom have direct control over the camera or character movement in 3D space. Establishing this limitation on player freedom gives the game designer control for technical and aesthetic reasons.
David Crane – creator of Pitfall – described his reasoning behind 2.5D thusly on the kickstarter page for his game Jungle Adventure:
At first glance, 2.5D sounds like it is less than 3D. It is not - it is just a different way to assemble a 3D scene. For certain subjects, such as a side-view scrolling scene, 2.5D can be better. Let's compare the two:
• A full 3D game display shows objects in the world from multiple angles. You can walk around them to see the back. This technique makes movies like Avatar possible, but to look good in a game requires the fastest PC with the most expensive graphics card; or a high-end gaming console and a $30M budget. Without that power the images can be very blocky. Advantage: See the backs of objects. Disadvantages: Clunky graphics on all but very high-end gaming consoles.
• 2.5D is used where the world layout allows for viewing only the fronts of objects. The same 3D rendering system can be used (i.e. Unity), placing the objects into a 3D world, but the world objects present a single pre-rendered texture face to the camera. Because the image is pre-rendered before it goes into the game, it can be made photorealistic, looking like the real object.
Both 3D and 2.5D have their uses. Using 2.5D in a game (where it makes sense) is a trick to manage game performance on most average PCs.
In the past, the term "2.5D" was more commonly used to refer to a variety of pseudo-3D graphical techniques that attempted to simulate three-dimensional graphics without the use of 3D polygons, including techniques in older games that feature 2D character sprites within a 3D environment. The most common technique was sprite scaling, which included Super Scaler technology (for various Sega arcade games from Hang-On to Galaxy Force), Mode 7 (for various SNES games), ray casting (for first-person shooters such as Doom and Duke Nukem 3D), sprite-based third-person or first-person perspectives, and sprite-based stereoscopic 3D. Other forms of 2.5D included vector graphics, isometric graphics, parallax scrolling, pre-rendered graphics, and cel-shading.
Coincidentally, the term "2.5D" is also used in AI pathfinding, where that usage refers to an entity that uses a two-dimensional grid with connecting nodes to navigate areas which may change their height axes.