Impermanent Loss
Impermanent loss is the shortfall of a liquidity pool position compared with simply holding the same tokens.
A liquidity pool rebalances as prices move, selling the token that rises and buying the one that falls. When the two prices diverge, the pool position ends up worth less than holding: about 5.7% less if one token doubles against the other, 20% less at 4x.
The loss is only realised when you withdraw, and it shrinks if the prices return, hence the name. Trading fees earned by the pool can offset it. The impermanent loss calculator shows the loss and the fee yield needed to break even.