The instance or form of communication between identities and service providers, each with its own set or security processes and risks. Similar to "context" where the circumstances (where, when, how) of an authentication can influence its assurance level. For example; face-to-face, proxy/representative, legal documents, telephone, mail, on-line network, email, FTP, internet, world wide web, unusual location, unusual time of day.

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