Mission Critical

Any telecommunications or information system that is defined as a national security system (FISMA) or processes any information the loss, misuse, disclosure, or unauthorized access to or modification of, would have a debilitating impact on the mission of an agency. 

See the following under Mission Critical for detailed information: 
NIST SP 800-60 Vol. 1 Rev. 1. 
NIST SP 800-60 Vol. 2 Rev. 1.