Opus

Opus (abbreviated Op.) is a numbering system composers use to catalogue their works — Beethoven's Op. 27 No. 2 is the Moonlight Sonata, for example. It helps you find the exact piece you're looking for, especially when a composer wrote hundreds of works. When your teacher says 'play the Chopin Op. 9 No. 2,' now you know they mean his second Nocturne from his ninth published set.