Which term indicates a gradual decrease in speed in music?

The term that indicates a gradual decrease in speed in music is indeed “rallentando.” This term is often used in musical notation to instruct performers to slow down the tempo over a gradual period of time, allowing for a more expressive interpretation of the music. Rallentando typically suggests a smooth transition into a slower pace, enhancing the emotional impact of the piece.

While both "ritardando" and "rallentando" serve to decrease tempo, "rallentando" specifically connotes a more gradual and expressive slowing down, often used at the end of a phrase or section. In contrast, "accelerando" refers to an increase in speed, making it the opposite of what the question is asking. "Stretto," on the other hand, is a term that is typically used in the context of a musical composition, often involved in a passage where themes are stated in close succession, rather than relating to tempo changes. Therefore, "rallentando" is the most appropriate term to indicate a gradual decrease in speed in music.