Simple Knowledge Organization System
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SKOS stands for Simple Knowledge Organisation System. It's a simple language for representing a thesaurus, classification scheme, taxonomy, subject-heading system, or other type of controlled structured vocabulary in a machine-understandable format. SKOS is an RDF-based language, which means it can also be used to publish controlled structured vocabularies in the Semantic Web.
SKOS is a work in progress, and the main published documents (the 'SKOS Core Guide', the 'SKOS Core Vocabulary Specification', and the "Quick Guide to Publishing a Thesaurus on the Semantic Web') have W3C Working Draft status.