The process of establishing confidence of authenticity; in this case, the validity of a person’s identity and an authenticator (e.g., PIV Card or derived PIV credential).
Sources:
FIPS 201-3 under Authentication

  Security measures designed to establish the validity of a transmission, message, or originator, or a means of verifying an individual’s authorization to receive specific categories of information.
Sources:
NIST SP 800-59 under Authentication from CNSSI 4009

  Verifying the identity of a user, process, or device, often as a prerequisite to allowing access to resources in a system.
Sources:
NIST SP 800-12 Rev. 1 under Authentication from FIPS 200
NISTIR 7316 under Authentication

  Verifying the identity of a user, process, or device, often as a prerequisite to allowing access to resources in an information system.
Sources:
NIST SP 1800-10B under Authentication from FIPS 200
NIST SP 1800-21C under Authenticate
NIST SP 800-128 under Authentication from FIPS 200
NIST SP 800-137 under Authentication from FIPS 200
NIST SP 800-18 Rev. 1 under Authentication
NIST SP 800-30 Rev. 1 under Authentication from FIPS 200
NIST SP 800-39 under Authentication from FIPS 200
NIST SP 800-60 Vol. 1 Rev. 1 under Authentication from FIPS 200
NIST SP 800-60 Vol. 2 Rev. 1 under Authentication from FIPS 200

  To confirm the identity of an entity when that identity is presented.
Sources:
CNSSI 4009-2015

  The process a VPN uses to limit access to protected services by forcing users to identify themselves.
Sources:
NIST SP 800-113 under Authentication

  See Authentication.
Sources:
NIST SP 800-63-3 under Authenticate

  Verifying the identity of a user, process, or device, often as a prerequisite to allowing access to a system’s resources.
Sources:
NIST SP 1800-16B under Authentication from NIST SP 800-63-3
NIST SP 1800-16C under Authentication from NIST SP 800-63-3
NIST SP 1800-16D under Authentication from NIST SP 800-63-3
NIST SP 1800-17c under Authentication
NIST SP 800-63-3 under Authentication

  Provides assurance of the authenticity and, therefore, the integrity of data.
Sources:
NIST SP 800-67 Rev. 2 under Authentication

  A process that provides assurance of the source and integrity of information in communications sessions, messages, documents or stored data or that provides assurance of the identity of an entity interacting with a system.
Sources:
NIST SP 800-57 Part 2 Rev.1 under Authentication

  Verifying the identity of a user, process, or device, often as a prerequisite to allowing access to a system’s resources
Sources:
NIST SP 1800-17b under Authentication

  A process that provides assurance of the source and integrity of information that is communicated or stored or the identity of an entity interacting with a system.
Sources:
NIST SP 800-175B Rev. 1 under Authentication

  Note that in common practice, the term "authentication" is used to mean either source or identity authentication only. This document will differentiate the multiple uses of the word by the terms source authentication, identity authentication, or integrity authentication, where appropriate.
Sources:
NIST SP 800-175B Rev. 1 under Authentication

  A process that provides assurance of the source and integrity of information in communications sessions, messages, documents or stored data or that provides assurance of the identity of an entity interacting with a system. See Source authentication, Identity authentication, and Integrity authentication.
Sources:
NIST SP 800-57 Part 1 Rev. 5 under Authentication

  The process of verifying the identity of a user, process, or device, often as a prerequisite to allowing access to resources in an information system.
Sources:
NIST SP 1800-27B under Authentication from FIPS 200
NIST SP 1800-27C under Authentication from FIPS 200

  The process of establishing confidence in the identity of users or information systems.
Sources:
NISTIR 8149 under Authentication from NIST SP 800-63-3

  The process of verifying a claimed identity of a user, device, or other entity in a computer system
Sources:
NISTIR 4734 under Authentication

  the process of verifying the integrity of data that has been stored, transmitted, or otherwise exposed to possible unauthorized access.
Sources:
NISTIR 4734 under Authentication

  The process of proving the claimed identity of an individual user, machine, software component or any other entity.  Typical authentication mechanisms include conventional password schemes, biometrics devices, cryptographic methods, and onetime passwords (usually implemented with token based cards.)
Sources:
NISTIR 5153 under Authentication

  The process of establishing confidence in the claimed identity of a user or system
Sources:
NISTIR 7682 under Authentication

  Verifying the identity of a user, process, or device, often as a prerequisite for allowing access to resources in an information system.
Sources:
NISTIR 8301 under Authentication from FIPS 200

  measures the number of times an attacker must authenticate to a target in order to exploit a vulnerability.
Sources:
NISTIR 7864 under Authentication
NISTIR 7946 under Authentication