Diluendo
An Italian performance direction for a fading or dying away of the tone.
What it means
Diluendo, literally "diluting," denotes in music a gradual fading or dying away of the tone. Unlike a pure volume marking such as decrescendo, diluendo describes a quality of sound: the tone dissolves, becomes more transparent and weaker, until it disappears altogether, much like a color dissolving in liquid.
Musical use
Diluendo belongs to a group of Italian terms for fading away at the end of a piece or section, which also includes morendo (dying away) and perdendosi (losing itself). Diluendo should be distinguished from a plain decrescendo: decrescendo concerns volume alone, while diluendo and its related terms also suggest a dissolving of tone quality, often combined with a slight easing of tempo.
Performance practice
Shape a diluendo not merely as getting quieter but as a thinning of the sound: gradually ease the bow pressure or breath support, rather than simply reducing volume. Let the vibrato fade toward the end as well, so the tone truly seems to vanish rather than stopping abruptly.