An imbalance is any area where price moved so quickly in one direction that buying and selling were lopsided. The fair value gap is the most common example, but the broader idea is simply a stretch of chart where one side overwhelmed the other and left an inefficiency behind.
The efficiency principle
Markets prefer two-sided trading, where both buyers and sellers transact across a price range. A rapid, one-sided move skips that, so price often returns later to rebalance the area. This is the same mechanism behind FVGs, generalised to any lopsided run in price.
How to use it
Mark obvious imbalances as areas of interest where price may react on a return. Pair them with structure and liquidity for context; an imbalance on its own does not tell you direction, only that an inefficiency exists. And remember, not all imbalances get filled.
An imbalance is a one-sided move that left inefficiency — price often revisits it to rebalance, but not always.