A thesaurus (plural thesauri or thesauruses), sometimes called a synonym dictionary or dictionary of synonyms, is a reference work which arranges words by their meanings, sometimes as a hierarchy of broader and narrower terms, sometimes simply as lists of synonyms and antonyms. They are often used by writers to help find the best word to express an idea: Synonym dictionaries have a long history. The word 'thesaurus' was used in 1852 by Peter Mark Roget for his Roget's Thesaurus. While some works called "thesauri", such as Roget's Thesaurus, group words in a hierarchical hypernymic taxonomy of concepts, others are organized alphabetically or in some other way. Most thesauri do not include definitions, but many dictionaries include listings of synonyms. Some thesauri and dictionary synonym notes characterize the distinctions between similar words, with notes on their "connotations and varying shades of meaning". Some synonym dictionaries are primarily concerned with differentiating synonyms by meaning and usage. Usage manuals such as Fowler's Dictionary of Modern English Usage or Garner's Modern English Usage often prescribe appropriate usage of synonyms. Writers sometimes use thesauri to avoid repetition of words — elegant variation — which is often criticized by usage manuals: "writers sometimes use them not just to vary their vocabularies but to dress them up too much". Etymology The word "thesaurus" comes from Latin thēsaurus, which in turn comes from Greek (thēsauros) 'treasure, treasury, storehouse'. The word thēsauros is of uncertain etymology. Until the 19th century, a thesaurus was any dictionary or encyclopedia, as in the Thesaurus Linguae Latinae (Dictionary of the Latin Language, 1532), and the Thesaurus Linguae Graecae (Dictionary of the Greek Language, 1572). It was Roget who introduced the meaning "collection of words arranged according to sense", in 1852. History In antiquity, Philo of Byblos authored the first text that could now be called a thesaurus. In Sanskrit, the Amarakosha is a thesaurus in verse form, written in the 4th century. The study of synonyms became an important theme in 18th-century philosophy, and Condillac wrote, but never published, a dictionary of synonyms. Some early synonym dictionaries include: John Wilkins, An Essay Towards a Real Character, and a Philosophical Language and Alphabetical Dictionary (1668) are a "regular enumeration and description of all those things and notions to which names are to be assigned". They are not explicitly synonym dictionaries — in fact, they do not even use the word "synonym" — but they