Definition 

There are a  number of definitions of  'thesaurus'  provided by different experts and organisations. The most comprehensive one has been provided by the International Standards Organisation (ISO) as the basis of structure and functions of a thesaurus. In terms of functions, it states "a thesaurus is a terminological control device used in translating from the natural language of documents, indexers or users into a more constrained 'system language' (documentation language, information language)". In terms of structure, the Standard says,  "a thesaurus is a controlled and dynamic vocabulary of semantically and generically related terms which covers a specific domain of knowledge". 

In short, a thesaurus may be defined as a compilation of descriptors for use in an information retrieval system arranged in an alphabetical order and manifesting the various types of relationships existing  between the descriptors. An information retrieval thesaurus is a kind of semantic networking of concepts.