Final answer:
A security breach is when a threat agent bypasses or thwarts a system's security measures, such as unauthorized access to a website by exploiting a vulnerability.
Step-by-step explanation:
A security breach is the occurrence of a security mechanism being bypassed or thwarted by a threat agent. This term encompasses a wide range of activities that violate the intended security policy of a system, including unauthorized access to data, systems, or networks. To better understand the concept, let's consider an example where an attacker exploits a vulnerability in a website's authentication process to gain unauthorized access to user accounts. This scenario demonstrates a security breach of the website's defenses, allowing the threat agent to bypass the security measures that are supposed to protect user accounts.