The process of establishing an Identity to be used in a particular instance, by verifying an assertion (eg claiming to be the owner of a set of credentials). See Assertion. In principle the original issuer of a credential should be the one to authenticate it; in practice this may be problematic and methods have been devised to share the authentication process. Also see re-authentication, and mutual authentication.

Source: http://identityaccessman.blogspot.com/2006/08/identity-dictionary.html