A flash loan is an uncollateralized loan borrowed and repaid in a single blockchain transaction (13 seconds on Ethereum). Available on Aave, dYdX, and other protocols. Practitioners use flash loans to execute profitable strategies that would be impossible without them. Examples: 1. Arbitrage: Borrow 1000 ETH, buy on Uniswap at $1980, sell on Curve at $2000, repay loan, profit ~$20k.