Impermanent Loss
Impermanent loss is the value a liquidity provider gives up, compared to simply holding, when the two pooled tokens change price relative to each other.
In depth
The AMM formula sells the token that is rising and accumulates the one that is falling, so a provider ends up with less of the winner. The loss is "impermanent" because it reverses if prices return to where they started — but if they do not, it becomes real. Trading fees earned may or may not cover it.
Why it matters
It is the hidden cost of providing liquidity and a frequent surprise for new DeFi users.