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  A gateway that limits access between networks in accordance with local security policy.
CNSSI 4009-2015
NIST SP 800-12 Rev. 1 under Firewall
NISTIR 7621 Rev. 1 under Firewall

  An inter-network gateway that restricts data communication traffic to and from one of the connected networks (the one said to be “inside” the firewall) and thus protects that network’s system resources against threats from the other network (the one that is said to be “outside” the firewall).
NIST SP 800-82r3 under Firewall from RFC 4949

  A part of a computer system or network that is designed to block unauthorized access while permitting outward communication.
NIST SP 1800-27B under Firewall from NIST SP 800-152
NIST SP 1800-27C under Firewall from NIST SP 800-152
NIST SP 800-152 under Firewall

  A device or program that controls the flow of network traffic between networks or hosts that employ differing security postures.
NIST SP 800-41 Rev. 1 under Firewall

  A firewall is a device that has a network protection application installed to safeguard the network from intentional or unintentional intrusion. A firewall sits at the junction point or gateway between the two networks, usually a private network and a public network such as the Internet. The term “firewall” is derived from the process in which, by segmenting a network into different physical subnetworks, the firewalls limit damage that could spread from one subnet to another, acting in the same manner as fire doors or firewalls in automobiles.
NIST SP 800-35

  A system designed to prevent unauthorized accesses to or from a private network. Often used to prevent Internet users from accessing private networks connected to the Internet.
NISTIR 7316 under Firewall