Auth Code (EPP)
A per-domain transfer secret (also called EPP code or auth-info) that the losing registrar issues so a registrant can move the domain to a new registrar.
An auth code (sometimes called EPP code, auth-info, or transfer secret) is a short string set per-domain at the registry. To move a domain like `example.com` from one registrar to another, the registrant requests the auth code from the current (losing) registrar, hands it to the new (gaining) registrar, and the registry uses it to authorise the transfer. Auth codes should be high-entropy and rotated after every transfer or whenever the domain may have been exposed. Long-lived, predictable, or leaked auth codes are one route to unauthorised domain transfers, which is part of why registry lock exists.