It's Called the OED
The Oxford English Dictionary of the English Language
Probably the most famous English dictionary of all is the Oxford English Dictionary Second Edition, (OED), which calls itself “The definitive record of the English language." It is available as a subscription published by Oxford University Press, part of the University of Oxford. The value of the OED is the material that accompanies the normal entry material of a dictionary; most dictionaries provide pronunciation and syllables, parts of speech and meanings. The OED provides the etymology of a word, using quotations taken from the first time the word was found in print until the present day, giving variant spellings and historical context. There are about 2.5 million quotations from varied sources, including international works, with definitions for more than 500,000 words, and the total is growing. In the OED, a reader can find the entire history of a word, from the meaning and spelling when it was first used to the modern spelling and current usage.
In print, the Second Edition was published in 1989, and it's in 20 volumes, with three additional volumes published between 1993 and 1997. Online, a subscription to the dictionary provides you with access to all volumes in print, plus additions of new and revised words. The OED is currently undergoing a complete revision, with the aim of producing a Third Edition. Quarterly, from 1,000 to about 2,500 new words and revised definitions are added to the online edition. The online dictionary allows you to toggle between editions and revisions for any definitions which had alterations to see changes in meanings through the history of the dictionary.
The online version gives versatility and abilities to the reader which can not be found in the print volumes, including the ongoing inclusion of new words into the English language. The reader has hyperlink access to words used in definitions, and the ability to sort both words and quotations in an variety of different ways. You can ask to see all the examples of a word used during a single year, or all the words used in quotations from Charles Dickens with Greek roots. You can also toggle on and off any of the functions the online dictionary provides, from the full text of a definition to using a drop-down list to finding a particular section of a definition.
There is a very comprehensive tour of the OED online, showcasing and explaining the workings of the dictionary.
And, it has the ability, by using wild cards in searches throughout the dictionary, to be a marvelous crossword puzzle solver.
Access to the OED is expensive, for individuals, $295 a year, There are options to also pay for access on a monthly basis, and outside the Americas, on a weekly basis. However, institutions are also major users of the OED, and if a library or educational institution to which you belong subscribes to the OED, you can also gain access through them.
The Second Edition is also available on CD for approximately the same price as a yearly subscription, but numerous reviews of the software program show an enormous amount of frustration with the way the CD version was implemented, and the copy protection on the software.
The OED website has links to a number of resources provided by the Oxford University Press to aid English language learners.
They offer an RSS feed to the OED Word of the Day (see the sidebar on the right-hand side of the OED home page).