If something is imbalanced, the two sides are not even, and that unevenness is causing trouble or unfairness. You might hear an imbalanced diet, an imbalanced relationship, or an imbalanced system. It is the adjective that matches the noun imbalance: the noun names the problem, and imbalanced describes the thing that has it. Though actually, English speakers often prefer unbalanced in daily talk, especially about people and feelings (“He seemed unbalanced”). Imbalanced sounds a bit more like reports, science, and formal writing — about systems, numbers, and mixes rather than about a person’s mind. Both are understood; the room you are in decides which one people reach for.