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The term 2.5 dimensional model (2.5D model) is used for models that include calculation beyond a mere 2D model, but do not treat the third dimension in like manner. One common use is for such fluid dynamics models. 2.5D models represent a means of offering better simulation than a 2D model and less computation than a 3D model, an additional choice regarding how to make a model tractable, so as to run within available computing resources.
A typical layout for a 2.5D model is that it handles just a sample of the variation along one of the dimensions, one considered likely to have less substantial effects on the simulated fluid flow. In a model using rectangular coordinates, it might be a full 3D model of just a slab-shaped region cutting across the modeled fluid. The slab's borders might be "free", or contained, or might be joined, given the circumstances. In a model using cylindrical coordinates, a wedge-shape might be modeled. Other strategies include doing a few 2D models that are cuts across the fluid and using a simpler procedure than the full model to join them, or simply using a longer grid spacing along one dimension.
It is possible that a simulation (or calculation) is carried out in a transformed space, and the reduction of dimensions is not so straightforward as above. Also, in some cases, a simulation might be termed 4D, for including time, and such fractional dimensions could have a different meaning.
A typical procedure when developing a 2.5D model is to compare some sample runs with equivalent 2D and 3D models, to determine the relative costs and benefits of each strategy for the particular simulation.
The terms 2.5D and 2.5D model have uses in other fields for substantially different concepts. For example, within computer graphics, 2.5D is used for some shortcuts used for creating images of objects viewed at an angle, such as rendering an image of a box by simply stretching to reshape 2D images of each of the sides of the box visible in the image. The term 2.5D has also been used for stereo depiction.