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.
Source(s):
NIST SP 1800-17b under Multifactor Authentication
NIST SP 1800-17c under Multifactor 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).
Source(s):
NIST SP 1800-27B under Multifactor Authentication from CNSSI 4009-2015
NIST SP 1800-27C under Multifactor Authentication from CNSSI 4009-2015

  An authentication system that requires more than one distinct authentication factor for successful authentication. Multi-factor authentication can be performed using a multi-factor 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.
Source(s):
NISTIR 8149 under Multi-Factor Authentication from NIST SP 800-63-3