A collection of realms (domains) that have established trust among themselves. The level of trust may vary, but typically includes authentication and may include authorization.
Source(s):
NIST SP 800-95 under Federation from Web Services Federation Language (WS-Federation) (Version 1.0, 8 July 2003)
NIST SP 800-160 Vol. 2 [Superseded] from NIST SP 800-95

  A process that allows the conveyance of identity and authentication information across a set of networked systems.
Source(s):
NIST SP 800-63-3 under Federation

  A collection of realms (domains) that have established trust among themselves. The level of trust may vary but typically includes authentication and may include authorization.
Source(s):
NIST SP 800-160 Vol. 2 Rev. 1 from NIST SP 800-95