The word thesaurus is derived from 16th century New Latin, in turn from Latin thesaurus, from ancient Greek θησαυρός thesauros, "store-house", "treasury". Besides its meaning as a treasury or storehouse, it more commonly means a listing of words with similar, related, or opposite meanings (this new meaning of thesaurus dates back to Roget's Thesaurus). For example, a book of jargon for a specialized field; or more technically 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), film, sound recordings, machine-readable media, etc.
The first example of this genre, Roget's Thesaurus, was published in 1852, having been compiled earlier, in 1805, by Peter Roget. Entries in Roget's Thesaurus are not listed alphabetically but conceptually and are a great resource for writers.
Although including synonyms and antonyms, entries in a thesaurus should not be taken as a list of them. 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.
Thesaurus databases, created by international standards, are generally arranged hierarchically by themes and topics. Such a thesaurus places each term in context, allowing a user to distinguish between "bureau" the office and "bureau" the furniture. A thesaurus of this type is often used as the basis of an index for online material. The Art and Architecture Thesaurus, for example, is used to index the national databases of museums, Artefacts Canada, held by the Canadian Heritage Information Network (CHIN).
The most significant thesaurus project of recent years is the Historical Thesaurus of English (HTE), ongoing and based at the University of Glasgow. The HTE is a complete database of all the words in the second edition of the Oxford English Dictionary, arranged by semantic field and date. In this way, the HTE arranges the whole vocabulary of English from the earliest written records (in Anglo-Saxon) to the present alongside types and dates of use. It is the first historical thesaurus to be compiled for any of the world's languages and has been in progress since 1964. The HTE project has also produced the Thesaurus of Old English, derived from the whole HTE database (published in 1995, 2000 and now freely-available online here).