From my understanding isometric is actually a drafting term used to describe a 45° angle on the x, y and z from the SE as Rav's said but the term was simply incorporated into cartography as general term for this type of view. Seeing as isometric views are more of a contemporary style in my mind it's more of a general term. Perspective views can represent any angle I believe and from any quadrant and is primarily used to showcase landscape. I could be totally wrong in this assessment. But definitely I do know know isometric is x, y, z, 45°, SE, stupid drafting classes in college.