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