Video games which have a pseudo-3d graphical display, but are actually built from layers of 2d tiles. In these games, the viewing angle is usually fixed at some oblique value such as 45 degrees, so a square floor tile would appear as a diamond shape. Such games were first popularized in the early 1980's on computer systems such as the Sinclair Spectrum (or speccy), Commodore 64, Amstrad, Apple II, and others.
Isometric displays are most often used in strategy, simulation, and role-playing games such as Sim City and Diablo II. Such games are relatively simple to implement, are far less cpu intensive than true 3d games, and offer the benefits of both 2d and 3d displays.

Other examples of isometric games: