An attribute is any distinctive feature, characteristic, or property of an object that can be identified or isolated quantitatively or qualitatively by either human or automated means.
Information associated with a key that is not used in cryptographic algorithms, but is required to implement applications and applications protocols.
NIST SP 800-57 Part 1 Rev. 3
A claim of a named quality or characteristic inherent in or ascribed to someone or something. (See term in [ICAM] for more information.).
NIST SP 800-63-2
A distinct characteristic of an object often specified in terms of their physical traits, such as size, shape, weight, and color, etc., for real-world objects. Objects in cyberspace might have attributes describing size, type of encoding, network address, etc.
NIST SP 800-95
(Web Services Glossary - W3C Working Group Note 11 February 2004)