Impermanent Loss
The shortfall a pool depositor ends up with compared with keeping the two assets outside the pool, caused by their prices diverging.
When one asset in a pool moves relative to the other, arbitrage traders rebalance the pool by taking out the asset that rose and adding the one that fell, so the depositor ends up with proportionally more of the weaker asset. The gap against simply keeping both assets separately grows with the size of the divergence and is symmetric, appearing whichever asset moves. It is called impermanent because it reverses if relative prices return to where they started, but it becomes permanent the moment the position is withdrawn. Accumulated swap fees and any incentive rewards can offset it, which is why divergence loss must always be assessed net of fees rather than on its own.
In practice
A pool holding two stablecoins that both track the same currency shows almost no divergence loss, while a pool pairing a volatile asset with a stablecoin can show a substantial one.
The common misunderstanding
The name is misleading: the loss becomes permanent at withdrawal, and only fee income earned in the meantime can make up for it.