Firewall: A network security device that monitors and controls incoming and outgoing network traffic based on predetermined security rules. A firewall typically establishes a barrier between a trusted internal network and an untrusted external network, such as the Internet. Firewalls are designed to prevent unauthorized access to or from private networks and can come in the form of hardware, software, or a combination of both. They are a fundamental part of network security and can perform various functions, including packet filtering, stateful inspection, proxy service, and network address translation (NAT).