For the unenlightened, "isometric" can be defined as a form of graphical projection - more specifically, an axonometric projection. It's a method of visually representing three-dimensional objects in two dimensions, in which the three axes of space appear equally foreshortened. Clear?
But why do we care? Because in 1982, Sega produced one of those quantum leaps in gaming that are so rarely seen. By displaying games using an isometric viewpoint it was possible to build faux-3D worlds that gave the player an almost God-like view of what was happening. Like the first moving picture or the first live TV broadcast, isometric games took the arcade world into a new era, and Zaxxon was its genesis.
The back-story is an age old tale; Man creates robot, robot kills man, robot builds fortress, robot takes over the world, plucky young underdog in underpowered spaceship tries to kill robot. But the awe that gamers felt the first time they saw the Zaxxon cabinet, with its colourful Lego style graphics and huge flight stick controller aroused the kind of revolutionary excitement that arcades were made for.