What is the definition and origin of "imba"?

It comes via real-time strategy games, where one goal of the designers is to keep things balanced for all the various players; deviations from that goal are carefully scrutinized by the community that plays the game. More precisely, it comes from the regular complaint from certain types of gamers QQing over their losses — usually due to their inferior skill at the game — that the game's mechanics are broken or imbalanced.

Using imba, then, as a shortening of imbalanced, is a facetious way of mocking that attitude, by declaring even acknowledged essential parts of the game — such as the Hunter class in World of Warcraft, or some of its top players (usually insanely good ones) — imbalanced because in a way, if you don't know how to handle uber classes or uber players, they "break" the mechanics, albeit in a legal way.

It means unbalanced, and it comes from imbalance. The word is normally used as an adjective (this move is imba), not as a noun (*you should fix the imba of that move). I have no idea why it became imba rather than unba, which would have been more logical; but this is how I have seen it used with regard to games. I don't think I have ever seen it used elsewhere.

It is used to indicate that a certain move, feature, or property renders one choice far superior or inferior to another, in such a way as to force a player into the superior choice or out of the inferior choice, whereas normally a good game would give a player several equally valid paths to victory.