The word thesaurus is New Latin for treasure; coined in the early 1820's. Besides its meaning as a treasury or storehouse, it more commonly means a listing of words with similar or related meanings. For example, a book of jargon for a specialized field; or more generally a list of subject headings and cross-references used in the filing and retrieval of documents. (Or indeed papers, certificates, letters, cards, records, texts, files, articles, essays and perhaps even manuscripts.)
Although including synonyms, entries in a thesaurus should not be taken as a list of synonyms. The entries are also designed for drawing distinctions between similar words and assisting in choosing exactly the right word. Nor does a thesaurus entry define words. That work is left to the dictionary.
In Information Technology, a thesaurus represents a database or list of semantically orthogonal topical search keys. In the field of Artificial Intelligence, a thesaurus may sometimes be referred to as an ontology.