decrescendo

noun

1. An instruction to play gradually more softly.
2. A gradual decrease in volume or loudness of a piece of music.

   “The peace was ended by the sound of an approaching train, the steady exhaust beat that has its own appeal, and amid a cloud of steam a rebuilt "Scot" burst from under the bridge at the south end of the station, and roared past at high speed. A long line of lighted windows flashed past, and then, in decrescendo, the sound and fury passed northwards.”

verb

1. To gradually become quieter

adj

1. becoming quieter gradually.