Which term indicates a gradual slowing down of the tempo?

The term that indicates a gradual slowing down of the tempo is "ritardando." This Italian musical term is commonly used in sheet music to instruct performers to gradually decrease the speed at which a piece of music is played. It adds expressiveness to the music, allowing for a natural ebb and flow in the tempo, making the emotional delivery of the piece more impactful.

In contrast, the other choices refer to different musical concepts. "Pianissimo" indicates playing very softly, which does not pertain to tempo changes. "Mezzo piano" means moderately soft, again unrelated to tempo. Lastly, "tempo primo" directs the performer to return to the original tempo, rather than imply any slowing down. Understanding these distinctions is crucial for interpreting and performing music accurately.