How does ISO 19011:2011 define an audit?

ISO 19011:2011 defines an audit as a "systematic, independent and documented process" for obtaining audit evidence and evaluating it objectively against audit criteria.

The full definition: an audit gathers audit evidence — records, statements of fact, or other verifiable information — and evaluates it objectively to determine the extent to which the audit criteria (the set of policies, procedures or requirements) are fulfilled. The three keywords matter: systematic (planned method, not ad hoc), independent (the auditor is not assessing their own work), and documented (evidence and conclusions are recorded). Several methods may be used, and audits can be internal or external.

Go deeper:

• ISO 19011 — Wikipedia — Leitfaden für Audits von Managementsystemen: Auditprinzipien, Programm-Management, Durchführung.