Assurance

Grounds for confidence that the other four security goals (integrity, availability, confidentiality, and accountability) have been adequately met by a specific implementation. “Adequately met” includes (1) functionality that performs correctly, (2) sufficient protection against unintentional errors (by users or software), and (3) sufficient resistance to intentional penetration or by-pass.
SOURCE: SP 800-27

The grounds for confidence that the set of intended security controls in an information system are effective in their application.
SOURCE: SP 800-37; SP 800-53A

Measure of confidence that the security features, practices, procedures, and architecture of an information system accurately mediates and enforces the security policy.
SOURCE: CNSSI-4009; SP 800-39

In the context of OMB M-04-04 and this document, assurance is defined as 1) the degree of confidence in the vetting process used to establish the identity of an individual to whom the credential was issued, and 2) the degree of confidence that the individual who uses the credential is the individual to whom the credential was issued.
SOURCE: SP 800-63