Source: http://en.wikipedia.org/wiki/Security_clearance
A security clearance is a status granted to individuals allowing them access to classified information, i.e., state secrets, or to restricted areas after completion of a thorough background check. The term "security clearance" is also sometimes used in private organizations that have a formal process to vet employees for access to sensitive information. A clearance by itself is normally not sufficient to gain access; the organization must determine that the cleared individual needs to know the information. No one is supposed to be granted access to classified information solely because of rank, position, or a security clearance.