System compromise means an attacker has gained unauthorized access to, or control over, a device, account, or service. The compromise may be partial, such as access to one workstation or mailbox, or complete, with the attacker able to change settings, run commands, or move into connected systems. It is a practical indicator that normal trust boundaries have failed.
In cyber security, system compromise matters because even a single affected system can expose credentials, data, and internal pathways for lateral movement. Defenders look for suspicious logins, new admin activity, persistence mechanisms, and unusual network connections to confirm scope and contain the intrusion. In real attacks, ransomware operators and other intruders often start with one compromised endpoint or account, then use it to expand access, steal data, or disrupt services.