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