In music, which term describes the action of slowing down gradually?

The term that describes the action of slowing down gradually is "ritardando." In musical notation, ritardando indicates that the performer should gradually decrease the tempo of the piece. This technique is often used to convey a sense of relaxation or to lead into a more impactful moment in the music.

Ritardando specifically emphasizes a gradual slowing down, allowing performers to smoothly transition to a slower pace, which can enhance the emotional quality of the music.

Rallentando also means to slow down, but it is generally interpreted as a more pronounced or sudden reduction in tempo compared to ritardando. Ritenuto refers to a sudden decrease in speed, often used more dramatically than a gradual decrease. Scherzando, on the other hand, is an indication of a playful, joking manner and does not relate to tempo changes. Thus, ritardando is the most precise term for a gradual slow down in music.