Penetration Testing, or "pen testing," is an offensive security practice where a simulated cyberattack is conducted against a system, network, or application to identify vulnerabilities that could be exploited by malicious attackers. It involves identifying potential weaknesses, misconfigurations, and other security flaws that could lead to a security breach. Pen testing can be manual or automated, and it often employs various tools and techniques used by real-world attackers.