The term “isometric” has become a popular word to describe any video game with an angled top-down or off axis viewpoint. However, true isometric perspective is actually a very specific 2-D drawing technique used to simulate the appearance of 3-D objects.
Before the advent of real-time 3-D computer graphics, many game developers used an axonometric projection technique to fake the appearance of 3-D. Games using this technique were commonly referred to as having “isometric graphics”. However, due to the nature of computer graphics, most of these games did not technically use isometric, but a very similar projection called “dimetric”.
In lower resolution artwork in which individual pixels are more pronounced, the traditional Isometric angle of 30° from the horizontal can produce jagged and ugly visuals. So it is usually substituted with an angle of 26.6° that produces a uniform 1:2 pixel ratio line that follows a neat pattern. For this reason most 2D game artwork described as "isometric" is actually "dimetric".
Mixing it Up
As technology advanced and 3-D became more popular, some games began mixing isometric and 3-D. The Sims is one example of this; the environments were made entirely of 2-D isometric graphics, while the 3-D models of the Sims was rendered on top of it.
As 2-D games declined in popularity the term “isometric” started being applied to any game with an angled overhead perspective. The problem with applying the term “isometric” to any 3D game like this is that it has no relevance to the actual rendering techniques or viewpoint of the game whatsoever.
Most top-down or overhead games slightly angle the view to give you a slightly better perspective on the action, but this angle does not automatically make them "isometric". The best way to differentiate isometric perspective from other kinds of perspectives is to remember that isometric is primarily used to refer to 2-D games that simulate the appearance of 3-D!