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A measure of the strength of an authentication mechanism and, therefore, the confidence in it, as defined in [NIST SP 800-63-3] in terms of three levels: AAL1 (Some confidence), AAL2 (High confidence), AAL3 (Very high confidence).
A category describing the strength of the authentication process.
NIST SP 1800-17b under Authenticator Assurance Level
NIST SP 800-63-3