Impermanent Loss
The shortfall a liquidity provider suffers when pooled asset prices diverge.
As one asset in a pair moves relative to the other, the AMM rebalances the pool, leaving providers with more of the weaker asset and less of the stronger one. Compared with simply holding, that difference is a loss.
It is called impermanent because it reverses if prices return to the original ratio. If they do not, and you withdraw, the loss is entirely permanent.