Final answer:
A firewall is a security mechanism that modifies the environment to return systems to normal after an unwanted or unauthorized activity has occurred.
Step-by-step explanation:
A firewall is an example of a security mechanism that modifies the environment to return systems to normal after an unwanted or unauthorized activity has occurred.
For example, if a user tries to access a restricted website, the firewall can block the connection and notify the system administrator, thus preventing any further unauthorized access.
Other security mechanisms that can be used to restore systems to normal after an unwanted activity include intrusion detection systems, antivirus software, and data backup systems.