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.
Source(s):
CNSSI 4009-2015
from
IETF RFC 4949 Ver 2
NIST SP 800-12 Rev. 1
under Gateway
from
IETF RFC 4949 Ver 2