There is an understanding like isometric or 2.5D games. As I understand those are the same things. But I could not understand completely what is it. Is it about how the art for the game is drawn? I mean if art is drawn such that you see the game scene from the angle (visible from a top-down perspective or side view [from wiki]) and not perpendicular, is this isometric? Or there are some programming tricks that make game isometric, such as ability of the hero to move in 3 dimensions whereas it actually moves only in 2D like in old Nintendo games (Ninja Turtles or Double Dragon). Please explain this understanding.
Isometric games are indead games viewed from above, in such a way that the x, y and z axis are exactly 120 degrees from eachother. So an isometric game is a game viewed in an isometric projection: http://en.wikipedia.org/wiki/Isometric_projection (this source is very reliable :P).
So as you have already guessed, it is about the way the game is drawn.