An authentication domain is a federation of service providers (with at least one identity provider) that is configured using Access Manager.
An authentication domain is not a domain in the Domain Name System (DNS) sense of the word.
Before an authentication domain can be configured, the service providers must contractually agree to exchange authentication information using the Liberty Alliance Project specifications. After this circle of trust is established, an authentication domain can be configured using Access Manager and single sign-on can be enabled. Simply put, an authentication domain is the term used by Access Manager when configuring a circle of trust. See Concept of Trust for related information.