[1] : The Collaborative International Dictionary of English v.0.48

magnum opus \mag"num o"pus\ ma^g"nu^m =o"pu^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

[2] : WordNet (r) 2.0

magnum opus
     n : a great work of art or literature

Magnum Opus >> M Words
Magnum Opus, definition of term: Magnum Opus
magnum+opus_pag1.html

Magnum >> M Words
Magnum, definition of term: Magnum
magnum_pag1.html

Opus >> O Words
Opus, definition of term: Opus
opus_pag1.html

Opus Incertum >> O Words
Opus Incertum, definition of term: Opus Incertum
opus+incertum_pag1.html