Which of the following best defines 'firewall' in network security?

The term 'firewall' in network security is best defined as a system designed to prevent unauthorized access to or from a private network. This definition emphasizes the primary function of a firewall, which is to monitor and control incoming and outgoing network traffic based on predetermined security rules. It acts as a barrier between a trusted network and untrusted networks, ensuring that only authorized users can access the private network while blocking potential threats from external sources.

Firewalls can be implemented in both hardware and software forms, serving to protect sensitive data and maintain the integrity of the network. They can log traffic, alert administrators to potential breaches, and provide various levels of filtering to enhance overall security, making them a vital component in protecting digital environments.

Other options provided do not accurately describe the function of a firewall. For example, while a physical barrier is important for securing data centers, it is not the same as a firewall's role in managing network traffic. Enhancing internet speed and automatically deleting old data are unrelated to the objectives of a firewall in network security.