Opus. 
From my music dictionary...
opus, pl. opuses or opera
It's Latin for "work" and is a term used for a musical work or
musical composition and generally used to indicate, by number, the
order of a composer's works, beginning with the first (opus 1) and
continuing to the last, or to distinguish compositions that have
the same title.
In practice, however, opus numbers frequently refer not to the
order of writing but to the order of publication. Also, some
composers assign opus numbers only to those compositions they
consider important, leaving the less important (in their view)
unnumbered.
A single opus may include one or several compositions; in the
latter case the pieces are usually related in some way, such as
being published at the same time, or being written as a series. For
example, Beethoven's opus 18 consists of six string quartets; they
are referred to as Quartet op. 18, no. 1; op. 18, no. 2 etc.
The word "opus" is often abbreviated op. or Op. and the plural form, opp.
