Online Dictionaries & Thesauri 

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General Dictionaries

Newbury House Online Dictionary of American English
"Provides simple and clear definitions, ... sample sentences, idioms, and a wide array of cultural facts and figures."

Oxford English Dictionary Online
"The Oxford English Dictionary (2nd edition) is the accepted authority on the evolution of the English language over the last millennium. It is an unsurpassed guide to the meaning, history, and pronunciation of over half a million words, both present and past. It traces the usage of words through quotations from a wide range of international English language sources, from classic literature and specialist periodicals to film scripts and cookery books."

Webster's Revised Unabridged Dictionary

 Specialized Dictionaries & Thesauri

Oxford Reference Online
Specialized topic dictionaries and reference titles across the full subject spectrum, which can be searched individually or as a whole.

The Alternative Dictionary
A dictionary of international slang.

Acronym Finder
Search for the meaning of over 79,000 acronyms.

Glossary of Musical Terms
General glossary of musical terms.

Online Medical Dictionary
Contains definitions of terms relating to medicine and science.

Rhyming Dictionary
Find rhymes, synonyms, anonyms, and more.

TechDictionary.com
Computer, Internet, file name extensions, and html tags are all included in this technology dictionary.

The World Wide Web Acronym and Abbreviation Server
Search for acronyms or words used in acronyms.

Roget's Thesaurus
Provides synonyms and antonyms.

Getty Thesaurus of Geographic Names
 

Language Dictionaries

ALLwords
A dictionary with all definitions in English, but with the ability to search for a word in English, French, German, Dutch, Italian, and Spanish. Additionally, there is pronunciation file for each English word.

The Internet Dictionary Project
Launched in 1995 in an effort to provide free translating dictionaries to the Internet community, the Internet Dictionary Project now includes translations to and from English in the following languages:  French, German, Italian, Latin, Portuguese, and Spanish.

Afrikaans/English - English/Afrikaans
Contains about 4,000 terms.

American Sign Language
Thousands of video clips demonstrating ASL.

Cheyenne/English - English/Cheyenne
Offered by the Cheyenne Language Web Site.

Chinese Character Genealogy
 An Etymological Chinese-English - - English-Chinese Dictionary. It also has a comprehensive list of Chinese studies.

Estonian/English - English/Estonian Dictionary
The Institute of Baltic Studies offers translations for more than 17,000 words.

Free Dictionaries.com
Translates to and from English using 15 major languages.

French-English - English French Dictionary

German/English - English/German Dictionary

Italian/English - English/Italian Dictionary

Japanese/English - English/Japanese Dictionary
Japanese word results presented in Japanese characters.

Russian/English - English/Russian Dictionary

Spanish/English - English/Spanish Dictionary

Swedish/English Dictionary - English/Swedish Dictionary
 

Dictionary Meta-Sites

Language and Translation Dictionary Center
Offered by Martindale's Desk Reference, this site includes many indigenous languages of North America

OneLook Dictionaries
The ultimate Internet dictionary lookup site. Search 475 general and specialized English dictionaries simultaneously, or select the ones you want to search.

YourDictionary.com
The ultimate Internet directory of "language products and services company that maintains the most comprehensive and authoritative language portal on the web with more than 2500 dictionaries and grammars" representing over 300 languages.   Graphically pleasing, easily navigable, with the following sections: Languages (dozens); Multilingual; Specialized, Thesauri and other Vocabulary Aids.

Word2Word
Word2Word links hundreds of dictionaries and glossaries that translate between the worlds major languages and dialects.  There is also a large index of online language courses and grammars as well as an index for many free online translation sites.