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