Impermanent loss
The shortfall a liquidity provider suffers versus simply holding, because the pool sells the token that rises and buys the one that falls.
If a provider deposits ETH and USDC and ETH doubles, arbitrage drains ETH from the pool and the provider ends up with less ETH and more USDC than they deposited — worth less than holding would have been. The loss is "impermanent" only if the price returns to the entry level. Fees are meant to compensate; they do on pairs that trade heavily and move little, and often do not on volatile tokens.