The Collaborative International Dictionary of English v.0.48:
magnum opus \mag"num o"pus\ (m[a^]g"n[u^]m [=o]"p[u^]s), n.
1. A great work of art or literature.
2. The greatest work or achievement of a particular artist,
writer, or other individual.
Inspired by this milieu, [Max Stirner] wrote his
magnum opus The Ego and Its Own, which was published
in November 1844. --Svein Nyberg
(The Small Max
WordNet (r) 3.0 (2006):
n 1: a great work of art or literature