Assurance definition in Computer Security terms:

Acronym(s): None

Definition(s): The grounds for confidence that the set of intended security controls in an information system are effective in their application.
Source(s): CNSSI 4009-2015 (Adapted from NIST SP 800-27 Rev. A)
NIST SP 800-137 (NISTIR 7298)
NIST SP 800-37 Rev. 1

Grounds for confidence that the other four security goals (integrity, availability, confidentiality, and accountability) have been adequately met by a specific implementation.