OpenID is an open, decentralized standard for user authentication and access control, allowing users to log on to many services with the same digital identity. It is an SSO method of access control. As such, it replaces the common login process that uses a login name and a password, by allowing a user to login once and gain access to the resources of multiple software systems.

An OpenID is in the form of a unique Uniform Resource Locator (URL), and is authenticated by the user's 'OpenID provider' (that is, the entity hosting their OpenID URL). The OpenID protocol does not rely on a central authority to authenticate a user's identity. Since neither the OpenID protocol nor web sites requiring identification may mandate a specific type of authentication, non-standard forms of authentication can be used, such as biometrics, or ordinary passwords.