The Oxford English Dictionary, often referred to as the OED, is a landmark reference work in English word usage and origins. The dictionary, which is the size of an encyclopedia, not only has every word and definition that the compilers could identify, but examples from literature of usage of each word including the oldest example the dictionary's compilers could find and representative examples throughout the word's history. It was compiled mostly by a group of dedicated scholars with the help of thousands of volunteers in the latter part of the 19th and the first part of the 20th century, requiring decades to complete.
The Oxford English Dictionary's amazing story is told in the books The Professor and the Madman and The Meaning of Everything: The Story of the Oxford English Dictionary.
The Oxford English Dictionary, 2nd Edition, is an updated edition brought out in 1989.