Which term describes the sound of dying or fading away in music?

Morendo is the term that signifies a gradual dying away or fading of sound in music. It derives from the Italian word meaning "dying," reflecting a sense of both decrease in volume and an emotional depth associated with the music's expression. When a piece or passage is marked with morendo, it typically suggests that the performer should not only decrease the loudness but also imbue the sound with an air of inevitability, as if the music is coming to a gentle close.

Decrescendo, while it does indicate a decrease in volume, does not specifically carry the same connotation of a complete fading away, as it may simply suggest a reduction in loudness without the implied emotional resonance. Pianissimo refers to a very soft dynamic level, focusing on volume rather than the act of fading away. Fermata indicates a pause or hold on a note or rest, hence it does not relate to the concept of dying away. Thus, morendo is the most appropriate term to describe this specific musical effect.