A thesaurus (plural thesauri or thesauruses) or synonym dictionary is a reference work for finding synonyms and sometimes antonyms of words. 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
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 A Dictionary of Modern English Usage often prescribe appropriate usage of synonyms.
Thesauri are sometimes used to avoid repetition of words, leading to 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".
The word "thesaurus" comes from Latinth?saurus, which in turn comes from Greek (th?sauros) 'treasure, treasury, storehouse'. The word th?sauros is of uncertain etymology.
Roget's Thesaurus, first compiled in 1805 by Peter Mark Roget, and published in 1852, follows John Wilkins' semantic arrangement of 1668. Unlike earlier synonym dictionaries, it does not include definitions or aim to help the user to choose among synonyms. It has been continuously in print since 1852, and remains widely used across the English-speaking world. Roget described his thesaurus in the foreword to the first edition:
It is now nearly fifty years since I first projected a system of verbal classification similar to that on which the present work is founded. Conceiving that such a compilation might help to supply my own deficiencies, I had, in the year 1805, completed a classed catalogue of words on a small scale, but on the same principle, and nearly in the same form, as the Thesaurus now published.
Roget's original thesaurus was organized into 1000 conceptual Heads (e.g., 806 Debt) organized into a four-level taxonomy. For example, liability was classed under V.ii.iv: Class five, Volition: the exercise of the will; Division Two: Social volition; Section 4: Possessive Relations; Subsection 4: Monetary relations. Each head includes direct synonyms: Debt, obligation, liability, ...; related concepts: interest, usance, usury; related persons: debtor, debitor, ... defaulter (808); verbs: to be in debt, to owe, ... see Borrow (788); phrases: to run up a bill or score, ...; and adjectives: in debt, indebted, owing, .... Numbers in parentheses are cross-references to other Heads.
The book starts with a Tabular Synopsis of Categories laying out the hierarchy, then the main body of the thesaurus listed by Head, and then an alphabetical index listing the different Heads under which a word may be found: Liable, subject to, 177; debt, 806; duty, 926.
Some recent versions have kept the same organization, though often with more detail under each Head. Others have made modest changes such as eliminating the four-level taxonomy and adding new heads: one has 1075 Heads in fifteen Classes.
Some non-English thesauri have also adopted this model.
Other thesauri and synonym dictionaries are organized alphabetically.
Some designate a principal entry for each concept and cross-reference it.
A third system interfiles words and conceptual headings. Francis March's Thesaurus Dictionary gives for liability: CONTINGENCY, CREDIT-DEBT, DUTY-DERELICTION, LIBERTY-SUBJECTION, MONEY, each of which is a conceptual heading. The CREDIT--DEBT article has multiple subheadings, including Nouns of Agent, Verbs, Verbal Expressions, etc. Under each are listed synonyms with brief definitions, e.g. "Credit. Transference of property on promise of future payment." The conceptual headings are not organized into a taxonomy.
Benjamin Lafaye's Synonymes français (1841) is organized around morphologically related families of synonyms (e.g.logis, logement), and his Dictionnaire des synonymes de la langue française (1858) is mostly alphabetical, but also includes a section on morphologically related synonyms, which is organized by prefix, suffix, or construction.
Before Roget, most thesauri and dictionary synonym notes included discussions of the differences among near-synonyms, as do some modern ones.
A few modern synonym dictionaries, notably in French, are primarily devoted to discussing the precise demarcations among synonyms.
Some include lists of objects by category, e.g. breeds of dogs.
The Historical Thesaurus of English (2009) is organized taxonomically, and includes the date when each word came to have a given meaning. It has the novel and unique goal of "charting the semantic development of the huge and varied vocabulary of English".
Bilingual synonym dictionaries are designed for language learners. One such dictionary gives various French words listed alphabetically, with an English translation and an example of use. Another one is organized taxonomically with examples, translations, and some usage notes.
Information science and natural language processing
Gerda Hassler, "Lafaye's Dictionnaire des synonymes in the History of Semantics" in Sheli Embleton, John E. Joseph, Hans-Josef Hiederehe, The Emergence of the Modern Language Sciences, John Benjamins 1999, ISBN1556197594, p. 1:27-40
Werner Hüllen, "Roget's Thesaurus, deconstructed" in Historical Dictionaries and Historical Dictionary Research, papers from the International Conference on Historical Lexicography and Lexicology, University of Leicester, 2002, Max Niemeyer Verlag 2004, ISBN3484391235, p. 83-94
Werner Hüllen, A history of Roget's thesaurus : origins, development, and design, Oxford University Press 2004, ISBN0199254729
Gertrude E. Noyes, "The Beginnings of the Study of Synonyms in England", Publications of the Modern Language Association of America (PMLA)66:6:951-970 (December 1951) doi:10.2307/460151JSTOR460151
Eric Stanley, "Polysemy and Synonymy and how these Concepts were Understood from the Eighteenth Century onwards in Treatises, and Applied in Dictionaries of English" in Historical Dictionaries and Historical Dictionary Research, papers from the International Conference on Historical Lexicography and Lexicology, University of Leicester, 2002, Max Niemeyer Verlag 2004, ISBN3484391235, p. 157-184
^Roget, Peter. 1852. Thesaurus of English Language Words and Phrases.
^ abAmerican Heritage Dictionary of the English Language, 5th edition, Houghton Mifflin Harcourt 2011, ISBN9780547041018, p. xxvii
^ abcB. Lafaye, Dictionnaire des synonymes de la langue française, Hachette 1869, 3rd edition
^John Wilkins, An Essay Towards a Real Character, and a Philosophical Language, London 1668 full text
^John Wilkins, William Lloyd (anonymously), An Alphabetical Dictionary Wherein all English Words According to their Various Significations, Are either referred to their Places in the Philosophical Tables, Or explained by such Words as are in those Tables, London 1668 full text
^Natascia Leonardi, "An Analysis of a Seventeenth Century Conceptual Dictionary with an Alphabetical List of Entries and a Network Definition Structure: John Wilkins' and William Lloyd's An Alphabetical Dictionary (1668)" in Historical Dictionaries and Historical Dictionary Research, papers from the International Conference on Historical Lexicography and Lexicology, University of Leicester, 2002, Max Niemeyer Verlag 2004, ISBN3484391235, p. 39-52
^Gabriel Girard, La Justesse de la langue françoise, ou les différentes significations des mots qui passent pour synonymes, Paris 1718, full text
^John Trusler (anonymously), The Difference between Words esteemed Synonyms, in the English Language; and the proper choice of them determined, London, 1766 full text
^Hester Lynch Piozzi, British Synonymy; or, an Attempt Regulating the Choice of Words in Familiar Conversation, Dublin 1794 full text
^James Leslie, Dictionary of the Synonymous Words and Technical Terms in the English Language, Edinburgh, 1806 full text
^George Crabb, English Synonyms Explained, in Alphabetical Order with Copious Illustrations and Examples Drawn from the Best Writers, 2nd edition, London 1818 full text
^Merriam-Webster's Dictionary of Synonyms : Choose Words with Precision, 1994, ISBN0877799067 uses an asterisk
^Henri Bertaud du Chazaud, Dictionnaire de synonyms et contraires, Le Robert "Les Usuels", 1998, ISBN2850364568
^Roger Boussinot, Dictionnaire des synonymes, analogies et antonymes, Bordas 1981, ISBN2040120092
Francis Andrew March, Francis A. March, Jr., March's Thesaurus and Dictionary of the English Language (issued under the editorial supervision of Norman Cousins), Doubleday, 1968, p. 598 full text, 1906 edition
^Pierre Benjamin Lafaye, Synonymes français, Paris 1841 full text
^Henri Bénac, Dictionnaire des synonymes, Hachette 1956, ISBN2010112199 (1982 edition)
^Christian Kay, Jane Roberts, Michael Samuels, Irené Wotherspoon, Historical Thesaurus of the Oxford English Dictionary, Oxford University Press 2009, ISBN9780199208999, p. ix
^R.E. Batchelor, M.H. Offord, Using French Synonyms, Cambridge University Press, 1993, ISBN0521372771
^Marie-Noëlle Lamy, The Cambridge French-English Thesaurus, Cambridge University Press, 1998, ISBN0521563488