2.5D is a very broad term, it can refer to any of a large number of methods to make things look more 3D while being 2D. Some more specific terms for this is "3/4 perspective" or "RPG perspective" (even though there's no perspective), or "RPG view".
Cyangmou has a pretty good explanation of the logic behind RPG perspective in
this tutorial. Make sure not to miss part 2, linked in the description.
The basic idea behind RPG perspective is that the y axis represents both depth and height, so you'll see both the fronts and tops of objects, but not the sides, bottoms, or backs. For a more natural look, you scale the depth and height by 3/4. Without the scaling, you get an oblique projection, which looks unnatural. Beyond that, it's mostly a matter of your taste as an artist. You can always fudge things if that makes them look better.