Latin for work. Opus numbers, written Op., roughly order a composer's published pieces.
Op. is short for opus, meaning work. Publishers numbered pieces as they came out, so Op. 67 is roughly the sixty-seventh publication. The numbers are useful for telling apart works with the same name (Beethoven wrote nine symphonies, but only one is Op. 67). They are not always in the order the music was written.
Also seen as: op., opus number