2.5D is when the graphics are three-dimensional but the gameplay is two-dimensional: the hero moves left and right, while the world around him is three-dimensional, with depth and a live camera. This approach combines the clarity of a classic platformer with the beauty of modern graphics. It's favored by platformers, metroidvanias, fighting games, and action games.
In 2D, everything is drawn flat; in 2.5D, the characters and world are three-dimensional, but movement is still mostly flat. The camera often rotates slightly, emphasizing depth.
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