imba (short for imbalanced) is derived from Strategy games (e.g. Warcraft) or MMOs. It is usually used to describe when a player or a character gets way too powerful to defeat that it becomes unfair/impossible for the opponents to play against him. Which results in everybody (mostly the opponents) to give up. Almost invincible. It is mostly used as a praise rather than a complain.
When not used in context to a game, it means elite, awesome, the best or unbeatable.
"After getting some early-game kills, his character became imba. He pawned us all."
in general reference:
"The Joker in the Dark Knight is IMBA!! .. he definately deserves an Oscar."
Tod: Yeah they should nerf him in the next version he's to good.
Imba can be applied to a subject in gaming (Weapon, army, race, role or job) which can be either too strong (Overpowered, or OP) or too weak (Underpowered, or UP).
Although the word IMBA can be applied to both OP and UP, it is mostly used to describe an in-game subject which is too strong.
A good example for imba is the job hunter in Ragnarok Online.
"Stop using Counterspells and Dark Rituals they're so imba."