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Erin McKean Redefines the Dictionary (a great video)

Google now has a definition search feature. In the search box type define: before the word you get a definition plus links to online dictionaries.

The major reference dictionary is the Oxford English Dictionary. It is available through GALILEO. 

Dictionary.com

YourDictionary.com

One Look Dictionaries

Merriam-Webster Online (The Student-friendly portal is Word Central)

Medline Plus Medical Dictionary

Note the link to a medical encyclopedia. This is a very authoritative resource on medical issues.

Chemistry and Environmental Dictionary

A Basic Dictionary of American Sign Language Terms

Little Explorers Dictionary

AlphaDictionary's Word Wizards Club

Rhyming Dictionary

Wordsmyth: The Educational Dictionary-Thesaurus

Write Express Online Rhyming Dictionary 

Symbols.com

Artlex Art Dictionary 

VisuWords

Say Hello to the World

This isn’t really a dictionary at all—but it is very interesting and there are great links to foreign language sites.