opus.

About opus

In music, the opus number is the "work number" that is assigned to a musical composition, or to a set of compositions, to indicate the chronological order of the composer's publication of that work. Opus numbers are used to distinguish among compositions with similar titles; the word is abbreviated as "Op." for a single work, or "Opp." when referring to more than one work. Opus numbers do not necessarily indicate chronological order of composition. For example, posthumous publications of a composer's juvenilia are often numbered after other works, even though they may be some of the composer's first completed works.