Thesaurus: The word thesaurus more commonly means a listing of words with similar, related, or opposite meanings. It is designed for drawing distinctions between similar words and assisting in choosing exactly the right word. A formal definition of a thesaurus designed for indexing is: a list of every important term (single-word or multi-word) in a given domain of knowledge arranged in a systematic order and manifesting various types of relationship existing between the terms; and a set of related terms for each term in the list. Some examples of thesaurus are Thesaurus of English Words & Phrases (ed. P. Roget); The Synonym Finder (ed. J. I. Rodale); Webster's New World Thesaurus (ed. C. Laird); etc.

The thesaurus consists of descriptors and non-descriptors. Descriptors are indexing terms consisting of one or more words, and representing always one and the same concept. Non-descriptors are terms which help the user to find the appropriate descriptor(s). They appear followed by a reference (USE operator) to the descriptor, which is the preferred term, and the only one which may be used for indexing. When a term is ambiguous, a “scope note” can be added to ensure consistency, and give direction on how to interpret the term. Naturally, not every term needs a scope note, but their presence is of considerable help in using a thesaurus correctly and reaching a correct understanding of the given field of knowledge.

Term relationships are links between terms that often describe synonyms, near-synonyms, or hierarchical relations. Hierarchical relationships are used to indicate terms which are narrower and broader in scope.

i) Related Term (RT): Synonyms and near-synonyms are indicated by a Related Term (RT). The way the term "Cybernetics" is related to the term "Computers" is an example of such a relationship.

ii) Broader Term (BT): A Broader Term (BT) is a more general term, e.g. “Apparatus” is a generalization of “Computers”.

iii) Narrower Term (NT): A Narrower Term (NT) is a more specific term, e.g. “Digital Computer” is a specialization of “Computer”.

BT and NT are reciprocals; a broader term necessarily implies at least one other term which is narrower. Thesaurus designers are generally careful to ensure that BT and NT indicate class relationships, as distinguished from part-whole relationships. Some thesauri also include Use (USE) and Used For (UF) indicators when an authorized term is to be used for another, unauthorized, term; for example the entry for the authorized term "Bird" could have the indicator "UF Aves". Reciprocally, the entry for the unauthorized term "Aves" would have the indicator "USE Bird".






Broader Term



Narrower Term



Related Term






Used For


Table 1: Symbols used in the word blocks of descriptors and non-descriptors