Penetration testing, also known as pen testing, is a type of security testing that is used to evaluate the security of a computer system, network, or web application. It involves simulating an attack on the system or application to identify vulnerabilities and weaknesses that could be exploited by attackers. 

Penetration testing typically involves the use of tools and techniques to probe the system or application for vulnerabilities, and may include activities such as network scanning, port scanning, and vulnerability assessment. It may also involve the use of manual testing techniques, such as manual code review and social engineering, to identify vulnerabilities. 

Penetration testing is an important part of a comprehensive security strategy, as it can help organizations to identify and address vulnerabilities before they can be exploited by attackers. It can be conducted by an in-house security team or by an external security consultant and may be performed on a one-time basis or as part of a regular testing program.