Mandatory Access Control (MAC) is a system in which rights and permissions are assigned in a completely centralized manner. MAC is most commonly found in military or national security contexts. MAC typically relies on a classification system to determine which users or processes are entitled to which system resources.
The Bell-LaPadula model, with emphasis on preventing leaks of classified information, is an example of MAC approach. MAC contrasts with Discretionary Access Control (DAC) in which multiple users can assign rights and privileges to system resources.
References:
http://www.cgisecurity.com/owasp/html/ch08s02.html
http://www.cisspzone.com/2011/04/bell-lapadula-access-control-model/