The term 'session hijacking' refers to a type of cyberattack where an attacker gains unauthorized access to a user's active session. This typically occurs when a user's session credentials, such as a session token or cookie, are stolen through methods like man-in-the-middle attacks or phishing. Once hijacked, the attacker can impersonate the user, potentially accessing sensitive information or performing unauthorized actions. Prevention measures include using secure connections (like HTTPS), adding multi-factor authentication, and implementing session expiration policies to protect against such vulnerabilities.