Unique data used to represent a person’s identity and associated attributes. A name or a card number are examples of identifiers. Note: This also encompasses non-person entities (NPEs).
Source(s):
CNSSI 4009-2015 from FIPS 201-1

  Unique data used to represent a person’s identity and associated attributes. A name or a card number are examples of identifiers.
Source(s):
NIST SP 800-171 Rev. 2
NIST SP 800-53 Rev. 5 from FIPS 201-2
NIST SP 800-79-2 under Identifier
FIPS 201 [version unknown] under Identifier
NIST SP 800-171 Rev. 1 [Superseded]

  The bit string denoting the identifier associated with an entity.
Source(s):
NIST SP 800-56B Rev. 2 under ID
NIST SP 800-56A Rev. 2 [Superseded] under ID
NIST SP 800-56B Rev. 1 [Superseded] under ID

  A bit string that is associated with a person, device or organization. It may be an identifying name, or may be something more abstract (for example, a string consisting of an Internet Protocol (IP) address).
Source(s):
NIST SP 800-56B Rev. 2 under Identifier

  A unique label used by a system to indicate a specific entity, object, or group.
Source(s):
NIST SP 800-171 Rev. 2
NIST SP 800-53 Rev. 5

  A bit string that is associated with a person, device, or organization. It may be an identifying name or may be something more abstract (e.g., a string consisting of an IP address and timestamp), depending on the application.
Source(s):
NIST SP 800-57 Part 1 Rev. 5 under Identifier

  information used to claim an identity, before a potential corroboration by a corresponding authenticator
Source(s):
NISTIR 8053 from ISO/TS 25237:2008

  Something (data) that identifies an assessment object or other entity of interest (like a defect check). In database terms, it is a primary or candidate key that can be used to uniquely identify the assessment object so it is not confused with other objects.
Source(s):
NISTIR 8011 Vol. 1 under Identifier

  A bit string that is associated with a person, device or organization. It may be an identifying name or a nickname, or may be something more abstract (for example, a string consisting of an IP address).
Source(s):
NIST SP 800-56A Rev. 2 [Superseded] under Identifier

  A bit string that is associated with a person, device or organization. It may be an identifying name, or may be something more abstract (for example, a string consisting of an Internet Protocol (IP) address and timestamp).
Source(s):
NIST SP 800-56B Rev. 1 [Superseded] under Identifier

  A bit string that is associated with a person, device or organization. It may be an identifying name, or may be something more abstract (for example, a string consisting of an IP address and timestamp), depending on the application.
Source(s):
NIST SP 800-57 Part 1 Rev. 3 [Superseded] under Identifier
NIST SP 800-57 Part 1 Rev. 4 [Superseded] under Identifier