The Oxford English Dictionary is the accepted authority on the evolution of the English language over the last millennium. It is a guide to the meaning, history, and pronunciation of over half a million words, both present, and past. It traces the usage of words through 2.5 million quotations from a wide range of international English language sources. The OED covers words from across the English-speaking world. It also offers etymological analysis and listings of variant spellings.
This online and free resource allows to quickly search for a species using either common name or scientific name. This is an excellent starting point when finding out information on a species.
AlphaDictionry.com Search 1065 Online English Dictionaries at Once!
Merriam-Webster Online Dictionary and Thesaurus The Merriam-Webster Online Dictionary is based on the print version of Merriam-Webster's Collegiate® Dictionary, Eleventh Edition.
Wordsmyth An online dictionary and thesaurus
This is another online and free resource brought to us by Thomson Reuters who are responsible for Web of Science, BIOSIS Previews, and Zoological Record databases. This index provides information about species taxonomy and name variation.