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An intermediate system (interface, relay) that attaches to two (or more) computer networks that have similar functions but dissimilar implementations and that enables either one-way or two-way communication between the networks.
CNSSI 4009-2015 from IETF RFC 4949 Ver 2
NIST SP 800-12 Rev. 1 under Gateway from IETF RFC 4949 Ver 2