A security or privacy control for an information system that is implemented at the system level and is not inherited by any other information system.
Sources:
NIST SP 800-37 Rev. 2
NIST SP 800-53 Rev. 5
from
OMB Circular A-130 (2016)
NIST SP 800-53A Rev. 5
from
OMB Circular A-130 (2016)
NIST SP 800-53B
from
OMB Circular A-130 (2016)