Flash loan
Uncollateralized loan repaid in the same transaction.
A unique DeFi trick: borrow millions USD with no collateral, but you must repay in the same on-chain transaction. If you don't, the entire tx reverts, the lender loses nothing. Legitimate uses: arbitrage between DEXes, refinancing positions. Unfortunately also a hacker favorite: in 2020-23 flash loans were used to attack bZx, Cream, Beanstalk (-$180M), Euler (-$200M).
In plain words
You borrow millions of dollars with no collateral, but must repay in the SAME blockchain transaction. If you don't, the whole operation reverts, the lender loses nothing. Legit use: price arbitrage across DEXes. Unfortunately hackers love this trick. Flash loans powered many of the biggest DeFi exploits in 2020-23.
Example
Beanstalk hack (April 2022): a $1B flash loan let the attacker hijack governance and steal $180M.