Lento is a music word. When a piece, or a part of a piece, is marked lento, you play it slowly. Musicians borrowed a set of speed words from Italian, and they use them all over the world: lento sits on the slow side, calmer and heavier than a walking speed. It often feels quiet, sad, or solemn, though it does not have to. You will see it written at the top of the music, in the same place as other speed marks. Outside music, English speakers almost never say lento; they just say slow. It can be used as a describing word (a lento passage) or as a name for a slow piece or slow section (a lento).