A dedicated gateway server with special security precautions on it, used to service external connections (typically from the public Internet). The firewall machine protects servers and networks hidden behind it from crackers.
The typical firewall is an inexpensive microprocessor-based Unix machine with no critical data, with public network ports on it, but just one carefully watched connection back to the rest of the cluster. The special precautions may include threat monitoring, call-back, and even a complete iron box keyable to particular incoming IDs or activity patterns.
The type of network and security environment of a firewall machine is often called a De-Militarised Zone (DMZ). It may contain other servers such as e-mail servers or proxy gateways - machines that need to be publicly accessible but also need some access to internal systems.
Also known as a (Venus) flytrap after the insect-eating plant.