"Based on the OED, the Historical Thesaurus of the Oxford English Dictionary contains almost every word in English from Old English to the present, allowing users to explore words connected in meaning throughout the history of the language. It provides a unique resource for scholars researching linguistic and literary history, the history of the language, social history, and more, and is a perfect complement to the OED itself, allowing the words in the OED to be cross-referenced and viewed in wholly new ways"
http://www.oup.com/online/ht/