What is Vulnerability

Based on wikipedia, Vulnerability refers to the inability to withstand the effects of a hostile environment. A window of vulnerability (WoV) is a time frame within which defensive measures are reduced, compromised or lacking. But, this explanation is a general sense.In computer security, a vulnerability is a weakness which allows an attacker to reduce a system's information assurance. Vulnerability is the intersection of three elements: a system susceptibility or flaw, attacker access to the flaw, and attacker capability to exploit the flaw. To exploit a vulnerability, an attacker must have at least one applicable tool or technique that can connect to a system weakness. In this frame, vulnerability is also known as the attack surface. -- Wikipedia

In a simple words,A security vulnerability is a weakness in a product that could allow an attacker to compromise the integrity, availability, or confidentiality of that product. -- Microsoft