Impermanent Loss
Plain English
A loss you experience when providing liquidity to a DEX pool if the price of your deposited tokens changes. You might have been better off just holding the tokens. It’s "impermanent" because it only becomes a real loss if you withdraw - if prices return to original levels, the loss disappears.
Technical
Divergence loss experienced by liquidity providers in AMMs when the price ratio of deposited assets changes from initial deposit. Caused by constant product formula - arbitrageurs rebalance pools to match external prices. Magnitude depends on price divergence magnitude and time. Mitigated by trading fees earned and LP incentives. Most severe with volatile pairs; minimal with stablecoin pairs.