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Junior Dictionary
http://www.enchantedlearning.com/Dictionary.html

Senior Dictionaries
//Dictionary// and Thesaurus - Free //Online// at Your //Dictionary//
YourDictionary is a free online dictionary that offers definition search from Webster's dictionary for word meanings and thesaurus.
www.yourdictionary.com/ -

Dictionary.com | Free //Online Dictionary// for English Definitions
Dictionary.com - the largest and most trusted free online dictionary. Quickly find accurate definitions and audio pronunciations of words.
dictionary.reference.com/ -

Cambridge //Dictionary Online//: Free English //Dictionary// and Thesaurus **...**===
Free Online English Dictionary, Thesaurus, Word of the Day, Pronouncing Dictionary and other language resources from Cambridge University Press.

OneLook //Dictionary// Search
Type in a word and search many dictionaries at once. Also has a reverse dictionary and translations.

Oxford //Dictionaries Online//
Dictionary by Oxford University Press. Answers questions on English grammar, spelling, and usage. Basic dictionary is free, premium content with ...
  • www.oxforddictionaries.com/

Memidex
Links to a large range of dictionaries and includes etymology references and audio pronunciation of words (both US English and British English).