Oxford English Dictionary
Print: REF PE 1625 .O87 1989 (20 vols) and REF PE 1625 .O92 1993 (3 vols)
The "OED," originally called New English Dictionary on Historical Principles [REF PE 1625 .N53 1888, 10 vols], is the definitive guide to the meaning, history, and pronunciation of over half a million words, tracing the usage of words from across the English-speaking world through quotations from international English language sources. Its intention was to record every word used in English since 1150 and trace it back through all its shifting meanings, spellings and uses to its earliest recorded appearance, plus at least one citation for each century of its existence. As a historical dictionary, the OED is very different from Dictionaries of current English, in which the focus is on present-day meanings. You’ll find present-day meanings in the OED, but also the history of individual words, and of the language.
The online version can be searched in a variety of ways: by word; phrase; definition; etymology; by author of the quotations which shows how the use of a word has changed over time. It is not exhaustive in its coverage of standard vocabulary and is limited in its treatment of slang, dialect, scientific, and technical terms, so other dictionaries should be used as well.