A thesaurus is a type of controlled vocabulary. It is considered the most elaborate form of vocabulary, as it contains a large amount of information. Terms in thesauri are related to each other by hierarchical, equivalent and/or associative relations. A hierarchical relation means that one term is considered broader or narrower than another, expressing for example a sort of relation: a guitar is considered a narrower term of a musical instrument because a guitar is a sort of musical instrument. It is a vertical relation. An equivalent relation means that several terms are considered equal, but one term is to be preferred to another. For example, house and dwelling are synonyms, but in a thesaurus one term will be preferred and the other will be alternative. This relation is horizontal. An associative relation represents non-direct relations: the term is not a narrower nor a broader term, nor is it a synonym, but there is a relation anyhow. Guitar can be a narrower term of musical instruments and guitar tabs can be a narrower term of sheet music. Even though they do not have a parent-child relationship, guitar can be linked to guitar tabs via an associative relation.

Terms in a thesaurus are considered unique and can have a unique identification number (reused in a URI). Their meaning and use are described in scope notes.

In the SKOS-model, developed by the World Wide Web Consortium (W3C), terms in a thesaurus are considered concepts. This is because, in SKOS, not the term is important, but its hierarchical, equivalent and associative relations, as well as all the additional information it contains, expressed in URIs. A term refers to the lexical string of syllables and vowels, whereas a concept refers to a unit of thought expressed in a formal computer language. Because of the formal characteristics of concepts, language barriers can be overcome when linking and retrieving resources (See: Athenawiki - Definitions).

Explore further the Linked Heritage learning object: Terminology.

See also: Controlled vocabulary, Simple Knowledge Organization System (SKOS), Terminology, Terminology Management Platform (TMP)

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