An authentication system that requires more than one distinct authentication factor for successful authentication. Multifactor authentication can be performed using a multifactor authenticator or by a combination of authenticators that provide different factors. The three authentication factors are something you know, something you have, and something you are.
Sources:
NIST SP 1800-17b under Multifactor Authentication
NIST SP 1800-17c under Multifactor Authentication

  An authentication system or an authenticator that requires more than one authentication factor for successful authentication. Multi-factor authentication can be performed using a single authenticator that provides more than one factor or by a combination of authenticators that provide different factors.
Sources:
NIST SP 800-53 Rev. 5 under multi-factor authentication

  The three authentication factors are something you know, something you have, and something you are. See authenticator.
Sources:
NIST SP 800-53 Rev. 5 under multi-factor authentication

  Authentication using two or more different factors to achieve authentication. Factors include something you know (e.g., PIN, password), something you have (e.g., cryptographic identification device, token), or something you are (e.g., biometric). See authenticator.
Sources:
NIST SP 800-172 under multi-factor authentication

  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).
Sources:
NIST SP 1800-27B under Multifactor Authentication
NIST SP 1800-27C under Multifactor Authentication from CNSSI 4009-2015

  Authentication using two or more different factors to achieve authentication. Factors include something you know (e.g., PIN, password), something you have (e.g., cryptographic identification device, token), or something you are (e.g., biometric). See authenticator.
Sources:
NIST SP 800-171r3 under multi-factor authentication

  An authentication system that requires more than one distinct type of authentication factor for successful authentication. MFA can be performed using a multi-factor authenticator or by combining single-factor authenticators that provide different types of factors.
Sources:
NIST SP 800-63-4 [