The word thesaurus is derived from 16th century New Latin, in turn from Latin thesaurus, from ancient Greek 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.
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Thesauri :: Reference Tools
Rhyming :: Dictionaries

American Society of Indexers - Thesauri Online - Annotated directory.
Bartleby.com: Reference - Includes Roget's International Thesaurus of English Words and Phrases and Houghton-Mifflin's Roget's II: The New Thesaurus, third edition.
Meta Description: [ Bartleby.com combines the best of both contemporary and classic reference works into the most comprehensive public reference library ever published on the web. ]
Germ - Searchable WordNet and Roget's Thesaurus with meaning trees.
HASSET - Provides direct term search followed by hierarchical and other types of browsing. Based on a UNESCO thesaurus.
Meta Description: [ ESDS/UKDA HASSET Thesaurus ]
Historical Thesaurus of English - Contains the vocabulary of English from the earliest written records to the present, with known recorded dates of usage, taken from the Oxford English Dictionary.
INGRID Library Thesaurus - Allows direct term search with results hyperlinked to broader and narrower search terms. Available in English and Estonian.
Lexical Freenet - Search for relationships between words and concepts that might never have occurred to you before.
Meta Description: [ A rich lexical toolbox,
this program allows you to search for rhymes and relationships
between words and concepts that might never have occurred to you before. ]
Phrases Thesaurus - A resource for writers. Enter a single word and it will return a list of phrases and sayings that are related to the word in some way. In English.
Meta Description: [ The Phrase Thesaurus, a writer's resource for generating ideas ]
Plumb Design Visual Thesaurus - An exploration of sense relationships within the English language. By clicking on words, you follow a thread of meaning, creating a spatial map of linguistic associations.
Meta Description: [ An online thesaurus and dictionary of over 145,000 words that you explore using an interactive map. It's a tool for people who think visually. The most fun you've ever had with words. The Visual Thesaurus was built using Thinkmap, a data visualization technology. ]
Roget's Interactive Thesaurus - Roget's Thesaurus of English words and phrases.
Meta Description: [ Free online Roget's Thesaurus of the English Language. ]
Roget's International Thesaurus of English Words and Phrases - Online publication of this popular thesaurus made available by Bartleby.com.
Meta Description: [ Mawson, C.O. Sylvester. 1922. Rogets International Thesaurus of English Words and Phrases ]
Roget's Thesaurus - As distributed by Project Gutenberg - E-text of Roget's Thesaurus No. Two, which is derived from the version of Roget's Thesaurus published in 1911.
Sybrina's Phrase Thesaurus - A creative phrase tool for creative writers of any genre including college students, people just learning English, people wanting to improve their communication skills, artistic professionals and others.
Meta Description: [ Sybrina's Phrase Thesaurus is an electronic reference tool for creative writers of any genre. Great for anyone with a need to compose unique, descriptive phrases. ]
WordNet - a Lexical Database for English - An on-line lexical reference system whose design is inspired by current psycholinguistic theories of human lexical memory. English nouns, verbs, adjectives and adverbs are organized into synonym sets, each representing one underlying lexical concept. Different relations link the synonym sets. From the Cognitive Science Laboratory, Princeton University
Wordsmyth Educational Dictionary-Thesaurus - Integrated english dictionary and thesaurus.
WWWebster Thesaurus - Merriam-Webster Thesaurus
Meta Description: [ Merriam-Webster provides a free online dictionary, thesaurus, audio pronunciations, Word of the Day, word games, and other English language resources. ]
Yahoo! Roget's II Thesaurus - Contains over 260,000 synonyms and cross-references. Features succinct word definitions and hyperlinked category index.
Meta Description: [ Roget's II: The New Thesaurus - find synonyms and word suggestions in this free online searchable thesaurus dictionary with over 260,000 synonyms and cross-references on Yahoo! Education. ]
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