Opus

In music, a prefix, used with a figure, to indicate the numbering of a composer's works, usually in chronological order.

It was at first a publisher's rather than a composer's device and in the early 18th century was used only for instrumental composers. Later in that century it began to become more general, being used for Haydn but not for Mozart, and from Beethoven onward it began to be used regularly, though the number of an Opus is not necessarily a guide to the date of its composition.