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