Oxford English Dictionary
The Oxford English Dictionary (OED; originally New English Dictionary) is the primary reference work on the English language and its evolution. It spans twenty printed volumes, but is also available on CD-ROM and over the Internet. It was originally planned by the Philological Society of London in 1857 to take ten years, but the first volume was not published until 1884. The completion of all the volumes took until 1928. In 1933, the first Supplement was published. In the 1980s, it was digitized using SGML.
The OED differs from many dictionaries in that for each word it includes several quotations that use the word, from printed sources dating back to 1,000 years ago. Its etymologies (explanations of word origins) are also much more detailed than those of most dictionaries.