The Collaborative International Dictionary of English v.0.48:

Opus \O"pus\, n.; pl. Opera. [L. See Opera.]
   A work; specif. (Mus.), a musical composition.
   [1913 Webster]

   Note: Each composition, or set of pieces, as the composer may
         choose, is called an opus, and they are numbered in the
         order of their issue. (Often abbrev. to op.)
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   Opus incertum. [L.] (Arch.) See under Incertum.
      [1913 Webster] Opuscle

The Collaborative International Dictionary of English v.0.48:

Incertum \In*cer"tum\, a.
   Doubtful; not of definite form.
   [1913 Webster]

   Opus incertum (Anc. Arch.), a kind of masonry employed in
      building walls, in which the stones were not squared nor
      laid in courses; rubblework.
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