D defs.my
Entry 2 senses Webster, 1913

magnum opus

/(măg"nŭm ō"pŭs)/ · mag·num o·pus · IPA /ˌmæɡnəm ˈoʊpəs/
01 n. A great work of art or literature.
  1. 1.
    A great work of art or literature.
  2. 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 Stirner Web Page, 1988 [http://www.math.uio.no/~solan/stirner/stirner.html])