Intrusion is an unapproved entry into a system, account, network, or other controlled environment. It can happen through stolen credentials, exploited software flaws, misconfigured access controls, or social engineering that tricks a user into opening a path inside. Unlike a simple scan or failed login, an intrusion implies that an attacker has crossed a security boundary and can begin interacting with protected resources.
In cyber security, intrusion matters because it is often the point where risk becomes impact. Once inside, an attacker may steal data, deploy malware, move laterally, or change settings to keep access hidden. Defenses focus on preventing, detecting, and containing intrusions with strong authentication, least privilege, patching, segmentation, logging, and intrusion detection systems. Fast investigation of unusual logins, process behavior, and network traffic helps teams spot and stop intrusions before they spread.