The set of physical and behavioral characteristics by which an individual is uniquely recognizable.
Sources:
FIPS 201-3 under Identity
NIST SP 1800-12b
NIST SP 800-79-2 under Identity

  The set of physical and behavioral characteristics by which an individual is uniquely recognizable. Note: This also encompasses non-person entities (NPEs).
Sources:
CNSSI 4009-2015

  The bit string denoting the identifier associated with an entity.
Sources:
NIST SP 800-56B Rev. 2 under ID

  The distinguishing character or personality of an entity.
Sources:
NIST SP 800-57 Part 1 Rev. 5 under Identity

  An attribute or set of attributes that uniquely describe a subject within a given context.
Sources:
NIST SP 1800-17b under Identity
NIST SP 1800-17c under Identity
NIST SP 800-63-3 under Identity

  Unique group element $0$ for which $x+0=x$ for each group element $x$, relative to the binary group operator $+$.
Sources:
NIST SP 800-186

  The unique representation of a subject engaged in an online transaction. A digital identity is always unique in the context of a digital service, but does not necessarily need to uniquely identify the subject in all contexts. In other words, accessing a digital service may not mean that the subject’s real-life identity is known.
Sources:
NIST SP 800-63-3 under digital identity

  The set of attribute values (i.e., characteristics) by which an entity is recognizable and that, within the scope of an identity manager’s responsibility, is sufficient to distinguish that entity from any other entity.
Sources:
NISTIR 7622 under Identity

  A set of attributes that uniquely describe a person within a given context.
Sources:
NISTIR 8149 under Identity from NIST SP 800-63-3