A review of contract source code by a third party against known vulnerability classes and the protocol’s intended behaviour.
Why it matters
An audit is evidence that someone competent looked, not a guarantee. Euler had been audited multiple times; the defect was a missing check on a path reviewers did not read as risk-bearing. The Curve incident could not have been found by source review at all, because the compiler was at fault.
Not advice. Definitions are for understanding, not instruction. Nothing here is financial, legal, tax or security advice.