A security clearance is a status granted to individuals allowing them access to classified information or to restricted areas, after completion of a thorough background check.
A background check or background investigation is the process of looking up and compiling criminal records, commercial records, and financial records of an individual or an organization.
Classified information is material that a government body claims is sensitive information that requires protection of confidentiality, integrity, or availability.
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The term "security clearance" is also sometimes used in private organizations that have a formal process to vet employees for access to sensitive information.
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A clearance by itself is normally not sufficient to gain access; the organization must also determine that the cleared individual needs to know specific information.
No one is supposed to be granted automatic access to classified information solely because of rank, position, or a security clearance.