A specific 3D perspective technically defined as having all 3 axes being exactly 120 degrees apart. In early video games, it is often slightly off to account for smoother pixel lines.
The isometric perspective is very popular as a method of depicting 3D environments in 2D graphics. It does not require graphics to scale with a change of perspective - the "left" and "right" sides will be of the same shape even if the object is mirrored. It also lends itself very easily to tiled environments. While much more common in 2D games, 3D games can display in an isometric perspective by simply focusing the camera to align the axes properly.
Limits of early graphics technology meant that the proper axis alignment did not match smoothly with how pixels were drawn. As such, most "isometric" games are in fact not isometric, but simply dimetric perspective (with the two "ground" axes being mirrored across the "height" axis). With anti-aliasing, modern 2D games can now employ true isometric perspective without
Questions under this tag deal with developing games that use isometric perspectives, whether they deal with a true isometric perspective or the just-near isometric perspective.