The word "imba" entered Russian-language gaming slang as a shortening of "imbalance," a calque of the English word imbalance. Originally it was used for whatever upsets the balance: a character, an item, or a move that is stronger than the developers intended and clearly outclasses the alternatives. When a player says "this hero is imba," they are claiming that picking anyone else is simply not worth it.
How the word is used today
Over time the meaning has expanded, and today the word has two main senses.
The literal, in-game sense. Imba is an objective skew in power. A hero, weapon, or strategy provides such an advantage that opponents have to either copy it or adapt to it. Balance is the relationship of strength among all available options: when none of them dominates, the game is considered balanced.
The figurative, colloquial sense. In everyday speech, "imba" has become a synonym for "cool," "great," "awesome." People may say it about a successful match, a beautiful play, or even something outside of games. Here no balance skew is implied at all — it is just an emotional assessment.
Derivatives have formed from the word: "imbovy" (very strong), "an imba build" (a set of items or abilities that gives a huge advantage), "to pick imba" (to choose the strongest character). There is also an English counterpart, OP, from overpowered, meaning "too powerful."
Imba versus simply a strong pick
Not every strong option is imba. A hero with a high win rate may simply be well played under the current conditions. People speak of imba when the advantage seems excessive and there are almost no counters. This is usually discussed by players and commentators, while the verdict is handed down by the developers: they change the numbers in patches, that is, game updates that include balance changes.
That is why imba status is almost always temporary. A patch can weaken ("nerf") an overly strong element or strengthen ("buff") weak ones, and yesterday's imba becomes an ordinary pick. In competitive games, this is why the metagame is constantly shifting — the set of the most effective strategies and picks at any given moment.
Why it matters in esports
For professional teams, imba is a matter not of emotions but of preparation. If the version of the game used at a tournament has a skewed hero or weapon, it starts being picked in almost every match, and opponents prepare answers. This is noticeable to viewers too: the draft, that is, the picking and banning of heroes before a match, turns into a struggle over who gets access to the strongest option.
A separate topic is the contested nature of the word itself. What some see as imba, others consider the result of skilled play, and such arguments never die down in communities: the arguments "it's broken" and "just learn to play against it" come up constantly.