Here are some online dictionaries:

http://www.oed.com/
Oxford English Dictionary

http://www.m-w.com/dictionary.htm

http://www.dictionary.com/
Dictionary.Com

http://www.yourdictionary.com/
Your Dictionary.Com

http://www.bartleby.com/61/
The American Heritage Dictionary of the English Language

http://www.s9.com/biography/
Biographical Dictionary
This dictionary covers more than 28,000 notable men and women who have shaped our world from ancient times to the present day. The dictionary can be searched by birth years, death years, positions held, professions, literary and artistic works, achievements, and other keywords.