Great news for the wordsmiths of the world! The Library now subscribes to the Oxford English Dictionary (OED) online.
"The Oxford English Dictionary (OED) is widely regarded as the accepted authority on the English language." It gives the meaning, history, and pronunciation of 600,000 words— past and present. The OED is a historical dictionary, so it is very different from other dictionaries. In addition to giving present-day meanings, you'll find the history of individual words, and of the language.