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.
Oxford English Dictionary (OED)