Vulnerability
A security weakness in a software program that puts the program or the complete computer at risk of malicious programs and users. For example, an Internet browser could have a vulnerability that if certain code is ran on a web page it causes the browser to crash or allows other users to read the files on your computer.
Once detected a vulnerability is resolved by the developer of the software program releasing a software update or patch that prevents or helps to prevent the vulnerability from being exploited.
Also see: Bug, Exploit, Security definitions