Magnum opus is a Latin phrase which literally means “great work.” A magnum opus is a work on a large scale which is widely regarded as an artist's pinnacle of achievement, often reflecting a lifetime of work. Proust's In Search of Lost Time, an epic cycle of seven novels, is an example of a magnum opus. It may take decades or even centuries for the magnum opus of a particular artist to be recognized or uncovered, as it is not uncommon for artists to die while completing their longest and most elaborate works.
The magnum opus is not necessarily the work which garners the most attention. In fact, in some cases the work is largely regarded as a failure during the artist's lifetime, with critics and members of the public alike expressing distaste for the piece, along with pity for the artist's failure. A magnum opus may also be viewed as obscure, challenging, and too difficult for most people to relate to. Sometimes, the identification of a magnum opus requires extensive reflection and study by professionals in the feel.
Some artists have in fact chafed at the promotion of work which they feel is of lesser value, arguing that their finest works are disregarded because of their length, complexity, or innovative nature. In the days of patronage in the arts, some artists felt forced to make generic, uninteresting work by their patrons, reserving their spare time for work on a magnum opus. Others actively concealed their great works during their lifetimes, for various reasons.
The difference between a magnum opus and other works by an artist is quite clear. A magnum opus is a work which stands out from the rest of someone's body of work because of its complexity, detail, length, and intensity. One could compare the difference to the distinction between a simple piano concerto and a composition for full orchestra; while both may be good, one is certainly larger, bolder, and more complex.
For those who enjoy being pedantic, there is some dispute over the pluralization of “magnum opus.” Some authorities suggest that “magnum opuses” is appropriate, while others prefer “magna opera.” The difference depends on how true one wants to be to the original Latin, with some people arguing that the correct Latin form should be preserved, though others have suggested that the phrase seems to have been absorbed into English, and that therefore the rules of English pluralization are sufficient.