Zones & imbalances
An order block that failed without a liquidity sweep: the failed swing's last candles become the new zone.
A mitigation block is an order block that failed without sweeping liquidity first. In a bearish one, price makes a high, pulls back, rallies again but fails to reach the old high (a failure swing), then breaks below the pullback low. The last down candle(s) before that failed rally are the mitigation block, and price often returns to them from below.
It is the twin of the breaker. Both are failed order blocks; the breaker swept the old high before failing, the mitigation block fell short. Remember it as: sweep means breaker, failure means mitigation.
In smart money concepts 'mitigation' also just means price returning to a zone. That is a different use of the word.
For education only, not financial advice.