Kerberos

Kerberos is a network-based authentication protocol useful for providing single sign-on to multiple servers.

Kerberos was developed at MIT, originally for Unix servers. Kerberos is also used, with extensions, in Windows Server products and in Active Directory.

See: RFC 4120.

References:

http://web.mit.edu/kerberos/

http://technet.microsoft.com/en-us/library/bb742516.aspx