What does Q clearance mean?

The United States Department of Energy offers Q clearance, which is a type of security clearance similar to Top Secret clearance but with access authorization to restricted data and special nuclear material. This clearance is primarily given to outside contractors who need access to specific sites, but may also be required for certain positions within the Department of Energy and the military. Contractors with Q clearance often require armed security escorts when accessing certain types of information, making it the highest type of clearance given to contractors.