An uncollateralised loan that must be borrowed and repaid within a single transaction, reverting entirely if repayment fails.
Why it matters
Flash loans are not themselves an exploit. They remove the capital requirement for an attack, letting anyone temporarily control enough value to manipulate a price, a governance vote or a liquidation. The Euler attack used one.
Not advice. Definitions are for understanding, not instruction. Nothing here is financial, legal, tax or security advice.