Impermanent Loss
The loss liquidity providers face when the price ratio of deposited tokens changes compared to when they deposited them. It is 'impermanent' only while funds remain in the pool and becomes permanent once withdrawn.
To put it in plain English
Like putting $100 in quarters and $100 in dimes into a shared jar, but when you withdraw them, quarters are now worth more - you'd have been better off just holding the quarters.