What is the term used for a musical passage that indicates to slow down gradually?

The term that indicates to slow down gradually is "rallentando." This musical directive instructs the performer to gradually decrease the tempo of the music, creating a feeling of relaxation or a gentle decrease in intensity as the piece progresses. This technique is often employed to convey a sense of farewell or to heighten emotional expression within a performance.

Ad libitum refers to a performer’s freedom to alter the tempo or rhythm of a passage, often resulting in a more spontaneous interpretation, but it does not specifically indicate a gradual slowdown. Accent indicates a note should be played with a stronger emphasis or attack, and ben marcato suggests a marked or emphatic style in playing but does not pertain to tempo changes. Thus, rallentando is the precise term that captures the essence of slowing down gradually in a musical context.