Magnum opus (plural: magna opera, also opus magnum / opera magna), from the Latin meaning "great work", refers to the largest, and perhaps the best, greatest, most popular, or most renowned achievement of a writer, artist, composer or craftsman.
Related terms 
Sometimes the term magnum opus is used to refer to simply "a great work" rather than "the great work" of a specific person.
In popular usage, the English-style plural magnum opuses may also be seen. Some reputable dictionaries such as the Oxford English Dictionary now regard magnum opuses as acceptable.
The term "magnum opus" is distinguished in usage from "masterpiece" by a requirement that it is a work on a large scale, and by the absence of a requirement that it is generally regarded as among the creator's most successful works.
See also 
|Look up magnum opus in Wiktionary, the free dictionary.|
- The American Heritage Dictionary of the English Language, Fourth Edition, Retrieved December 10, 2006, from Dictionary.com
- Bill Bryson, Troublesome Words, Third Edition published in Penguin Books 2002