The Oxford English Dictionary

The 20-volume Oxford English Dictionary 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. The OED has a unique historical focus. Accompanying each definition is a chronologically arranged group of quotations that trace the usage of words, and show the contexts in which they can be used. Other features distinguishing the entries in the Dictionary are authoritative definitions of over 500,000 words; detailed information on pronunciation using the International Phonetic Alphabet; listings of variant spellings used throughout each word's history; extensive treatment of etymology; and details of area of usage and of any regional characteristics (including geographical origins).