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.
      [WordNet 1.5]

   2. The greatest work or achievement of a particular artist,
      writer, or other individual.
      [PJC]

            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
                                                  Stirner Web
                                                  Page, 1988
                                                  [http://www.math.uio.no/~solan/stirner/stirner.html])
      [PJC]

WordNet (r) 3.0 (2006):

magnum opus
    n 1: a great work of art or literature