Flash loans

A flash loan is an uncollateralised loan that must be borrowed and repaid within a single transaction — if repayment (plus fee) doesn't happen before the transaction ends, the whole thing reverts and it's as if the loan never existed. This is only possible because of blockchain's atomic transaction semantics: the lender has no risk because execution is all-or-nothing. The use case is capital-efficient arbitrage and liquidations — you can borrow $50M from Aave, run a trade that closes a price gap across three DEXes, repay, and pocket the spread, all without needing a cent of your own capital. The dark side: flash loans power most of DeFi's biggest exploits, because an attacker can suddenly wield nine-figure capital to manipulate thin markets, governance votes, or oracle prices and be back to zero before anyone notices. The tool is amoral; what matters is what the protocols around it assume about adversaries.