Which term suggests a gradual slowdown in tempo?

The term that suggests a gradual slowdown in tempo is "Ritardando." This is a common musical direction indicating that the performer should gradually decrease the speed of the piece as it progresses. "Ritardando" is often abbreviated as "rit." and is employed to enhance the expressive quality of music, allowing for a feeling of relaxation or a sense of conclusion in certain passages.

While "Ritenuto" refers to a sudden slowing down or holding back of tempo, it does not imply a gradual decrease like "Ritardando." "Allargando" indicates a broadening of tempo that often includes a slight slowing down but also suggests an increase in volume or expression. "Piu," meaning "more," is typically used with other terms to indicate an increase in intensity or tempo, and does not pertain to a slowdown at all. Thus, "Ritardando" is the appropriate choice for indicating a gradual slowdown in tempo.