Technically, the term "isometric perspective" is only correct for some of the older games like some of SSIs where you got a perfect 30°/30° split looking down between x/y-axes and all axes have the same equal measure e.g. 1:1:1.
But it has been used so often it has naturalized in common usage.
Ultima Online was already called that even though the z-axis was distorted:
Technically it is the typical "Militärperspektive" (I don't know if there's an english name for it, but they used to draw military plans like that in good ol' Germany)
And some of the other games like Fallout and whatnot without a perfect degree split between x/y/z axes are actually axonometric, in the case of Fallout I believe cavalier oblique or other perspective types.
I think there was a thread on the Codex a bit back:
http://www.rpgcodex.net/forums/index.php?threads/just-what-is-isometric.69829/