Security Vulnerability

A security vulnerability is a weakness in a computer systems which can be exploited by malicious entities to disrupt the operation of your that system, steal information from that system, or trick users of that system into performing unsafe acts.

Example

Joe has a website addressed as www.example.com which is running an old version of their web server software, this version has a security vulnerability which allows hackers to take over Joe's site. A malicious user might use that vulnerability to steal information from Joe.