A thesaurus, 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:...to find the word, or words, by which [an] idea may be most fitly and aptly expressed

A reference work is a non-fiction work, such as a paper, book or periodical, to which one can refer for information. The information is intended to be found quickly when needed. Such works are usually referred to for particular pieces of information, rather than read beginning to end. The writing style used in these works is informative; the authors avoid opinions and the use of the first person, and emphasize facts.