In the context of information retrieval, the purpose of a thesaurus (plural: "thesauri") is to guide both an indexer and a searcher to select the same preferred term or combination of preferred terms to represent a given subject.
In the context of information retrieval, a thesaurus (plural: "thesauri") is a form of controlled vocabulary that seeks to dictate semantic manifestations of metadata in the indexing of content objects. A thesaurus serves to guide both an indexer and a searcher in selecting the same preferred term or combination of preferred terms to represent a given subject.