[1] : 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
see also:
opera opus incertum incertum 

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

incertum \incer"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
see also:
opus incertum 

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

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

Incertum >> I Words
Incertum, definition of term: Incertum
incertum_pag1.html

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