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Authentication using two or more factors to achieve authentication. Factors include: (i) something you know (e.g., password/personal identification number [PIN]); (ii) something you have (e.g., cryptographic identification device, token); or (iii) something you are (e.g., biometric).
NIST SP 1800-27B under Multifactor Authentication from CNSSI 4009-2015
NIST SP 1800-27C under Multifactor Authentication from CNSSI 4009-2015