Opus incertum

from 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.)
         [1913 Webster]

   {Opus incertum}. [L.] (Arch.) See under {Incertum}.
      [1913 Webster] Opuscle

from 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.
      [1913 Webster]