Vulnerability: A flaw or weakness in system security procedures, design, implementation, or internal controls that could be exercised (accidentally triggered or intentionally exploited) and result in a security breach or a violation of the system's security policy.

Example: A SQL Injection vulnerability exists in the PHP Nuke application

Example; Mac's are also susceptible to vulnerabilities don't you know ;-)

I'm using this for the Practical Risk Assessment Methodology:

 

Vulnerability: A flaw or weakness in system security procedures, design, implementation, or internal controls that could be exercised (accidentally triggered or intentionally exploited) and result in a security breach or a violation of the system's security policy.

• Example: A SQL Injection vulnerability exists in the PHP Nuke application
• Example: Lack of a viable backup solution means that we cannot restore a server

I would like to point out that I found the blog pretty interesting. It has made me familiar with the term vulnerability. Clear examples are also given which really goes a long way in conveying the idea. I would like to add something more on the SQL Injection vulnerability. It occurs most of the time when user input is incorrectly filtered or when it is not strongly typed. I think the best way to avoid this is by using parameterized statements or by carefully filtering the user input.