In music, fortissimo means very loud — louder than forte, which already means loud. On a score it is often written ff. It is a direction to the player or singer, not a number on a machine: how loud “very loud” is depends on the piece, the room, and the instrument. Though actually, like many Italian markings, it is a step on a ladder, not a scientific ceiling. Forte is strong; fortissimo is very strong; even louder markings exist (fortississimo, fff), but fortissimo is the one most learners meet. You may also hear it as a noun for a very loud passage: “the fortissimo at the end.” People say it in rehearsal in Italian or in English (“play that fortissimo”). It is at home in classical music talk; a pop musician might just say “much louder.”