Which term indicates a gradual slowing down in music?

The term that indicates a gradual slowing down in music is "ritard." This term specifically instructs the musician to decrease the tempo slowly over a passage, creating a sense of relaxation or conclusion as the piece progresses. The slowing down can enhance the emotional impact of the music, guiding the listener through a more reflective or softer phase of the piece.

In contrast, "a tempo" is a directive to return to the original speed after a section that may have been played at a different tempo. "Crescendo" refers to a gradual increase in volume, creating a build-up of sound rather than a slowing down. "Diminuendo," on the other hand, involves a gradual decrease in volume but does not imply a change in tempo. Each of these terms serves distinct purposes in musical instruction and performance, highlighting the unique role that "ritard" plays in conveying a slower emotional quality in music.