A vulnerability class in which a contract calls out to another contract before updating its own state, allowing the external contract to call back in and act on stale state.
Why it matters
One of the oldest and best-understood smart contract vulnerabilities, defended against with reentrancy guards. The Curve incident is notable because the guards were correctly written in source but emitted incorrectly by particular compiler versions.
Not advice. Definitions are for understanding, not instruction. Nothing here is financial, legal, tax or security advice.