Reference Monitor
The reference monitor is a virtual machine within a system that controls access to every object on the system, every time access is requested. Objects in the system are identified as any physical components, files, devices, or memory. It will allow access to an object only if it determines that the subject (individual, process, and so on) trying to access the object is allowed.
Reference monitors must perform this function every single time a request is made and the reference monitor itself must be secure. The reference monitor can only be deemed truly secure if it is small enough to allow for analysis and testing. Larger programs increase complexity, which in turn introduces many more unknown variables into the system. A small reference monitor model means that it can be tested and analyzed and that the results of those tests will likely be more precise.