If you've heard an orchestra play a final note that slowly fades till you almost can't hear it, they're playing it al niente. You can see the violinists still moving their bows, but you're not sure if any sound is coming out!
Al niente is Italian for "to nothing." It's an instruction to vocalists or instrumentalists to reduce the volume gradually to silence — so gradually that listeners can hardly tell at what point the music has stopped. This technique is often used at the end of a longish piece of music, and is a very effective way to close. Listeners are straining so hard to hear the note as it dies that their full attention is caught and they're holding their breath.