1. An academic model for enforcing access control for government and military. The model is based around the idea of mandator access control. The formal definition from TCSEC is: ...a means of restricting access to objects based on the sensitivity (as represented by a label) of the information contained in the objects and the formal authorization (e.g., clearance) of subjects to access information of such sensitivity In this definition, a "subject" is somebody (user) who wants access to an "object" (information, data file, system). The subject and object have different security levels. Objects (information, data, systems) are assigned security classification levels. A typical example would be: unclassified < confidential < secret < top-secret Subjects are assigned similar clearance levels that allow access to objects of similar level or below. For example, if you are a government employee with "secret" clearance level, you can access everything but "top-secret" information. A classification level such as "top-secret" will also include categories. For example, you may have a "secret" clearance for NATO information, and "top-secret" clearance for all matters pertaining to nuclear weapons. The system follows the principle of least privilege. Therefore, you would not be cleared to access top-secret NATO nuclear plans because your NATO clearance isn't high enough. From Hacking-Lexicon