A vulnerability is a weakness or gap in a system’s security that can be exploited by an attacker to gain unauthorized access or perform malicious actions. It can refer to a flaw in software, hardware, or a combination of both, that can be exploited to compromise the confidentiality, integrity, or availability of a system or its data. Vulnerabilities can be discovered through various means such as penetration testing, code reviews, or by being reported by external parties.

(Ed: written by ChatGPT; verified by jrivett.)