The dictionary meaning of the word 'thesaurus' is "a collection of words put in groups together according to likenesses in their meaning rather than an alphabetical list." However, in library and information science parlance the word thesaurus means an authoritative list showing terms which may and sometimes may not be used in a catalogue or index to describe concepts. Technically, a 'thesaurus' could be defined as "a compilation of words and phrases showing synonymous, hierarchical, and other relationships and dependencies, the function of which is to provide a standard vocabulary for information storage and retrieval systems" In a thesaurus, each term is usually given together with terms; which are related to it in a number of ways. The primary purpose of thesaurus is to exert terminology control in choosing proper headings in subject cataloguing and indexing. This concept is being extensively used since the 1950s in, many information retrieval systems and a variety of bibliographical tools. A number of standards have come into existence to provide guidance in the construction of thesauri. 'Root Thesaurus' published by the British Standards Institution is one of the best models. Apart from this, there are a number of thesauri such as TEST (Thesaurus of Engineering, Scientific and Technical Terms), and others concerned with INIS, INSPEC, AGRIS, etc., which are being used in information storage and retrieval systems.