Explanation:
A firewall is a network security device or software that monitors and controls incoming and outgoing traffic based on predetermined security rules. It acts as a barrier between a trusted network and potentially harmful external networks.

Firewalls filter traffic based on parameters like IP addresses, protocols, and ports. They are configured to permit or block specific data packets to protect against unauthorized access and cyber threats.

Types of firewalls include packet-filtering firewalls, stateful inspection firewalls, and next-generation firewalls (NGFWs) that incorporate advanced security features like deep packet inspection and threat intelligence.

Additional Information:
Firewalls are essential for protecting organizational networks from malware, hacking attempts, and data breaches. They are often deployed alongside intrusion detection systems (IDS) and intrusion prevention systems (IPS).

Modern firewalls offer advanced capabilities such as application layer filtering and user identity-based rules, enabling comprehensive security management.

Formal Definition:
A firewall is a network security system, either hardware or software-based, that monitors and controls incoming and outgoing traffic according to predefined security rules, protecting networks from unauthorized access and cyber threats.