Which term refers to the action of slowing down gradually in music?

The term that refers specifically to the action of slowing down gradually in music is "Ritardando." This term directs the performer to gradually decrease the tempo, creating a feeling of deceleration in the piece. It is commonly used to signal a transition in the music or to heighten emotional expression as the piece approaches a conclusion or a significant musical moment.

While "Ritenuto" also suggests a sudden reduction in tempo, its effect is more immediate rather than gradual. "Poco a poco," meaning "little by little," can refer to various changes in dynamics or tempo, but it does not specifically connote a slowing effect. "Adagio," on the other hand, is a term that indicates a slow tempo but does not imply a gradual slowing; instead, it establishes a specific speed for the music.