A faceted classification system allows the assignment of an object to multiple characteristics (attributes), enabling the classification to be ordered in multiple ways, rather than in a single, predetermined, taxonomic order. A facet comprises "clearly defined, mutually exclusive, and collectively exhaustive aspects, properties or characteristics of a class or specific subject". For example, a collection of books might be classified using an author facet, a subject facet, a date facet, etc.
Faceted classification is used in faceted search systems that enable a user to navigate information along multiple paths corresponding to different orderings of the facets. This contrasts with traditional taxonomies in which the hierarchy of categories is fixed and unchanging.
The colon classification developed by S. R. Ranganathan is an example of faceted classification applied to the physical world, specifically for the purpose of organizing library materials. In the colon classification system, a book is assigned a set of values from independent facets. It differs from traditional library classification schemas like the Dewey Decimal System and Library of Congress classification system, in which each document has a unique assignment in a single, hierarchically organized classification system.
Faceted classification systems are also distinct from folksonomies or other tagging systems that do not break out the tags into independent facets.
See also 
- Classification Research Group
- Controlled vocabulary
- Information architecture
- eXchangeable Faceted Metadata Language
- Taylor, A. G. (1992). Introduction to Cataloging and Classification. 8th ed. Englewood, Colorado: Libraries Unlimited.
- Colon Classification (6th Edition) by Dr. S.R. Ranganathan, published by Ess Publications, Delhi, India.
- How to Make a Faceted Classification and Put It On the Web - an article by William Denton
- How to Reuse a Faceted Classification and Put It On the Semantic Web
- Carl Linnaeus Invented The Index Card. ScienceDaily. Retrieved September 9, 2010, from http://www.sciencedaily.com /releases/2009/06/090616080137.htm - a mention of Linnaeus's use of faceted classification in the original index card system
- OpenMeta tagging system for the Mac - a (currently) flat system for making facets. (Requesting links for hierarchical tagging systems)