What term describes the musical direction to play gradually louder?

The term that describes the musical direction to play gradually louder is "crescendo." In musical notation, crescendo indicates a gradual increase in volume over a specified duration, allowing the performer to build intensity and expressiveness in their playing. This dynamic marking is essential in conveying emotion and creating a sense of movement within the music.

Stretto refers to a compositional technique where ideas are presented in close succession, often seen in fugues, and does not pertain to dynamics. Forte indicates a loud volume, but it does not imply a gradual increase; it is a static instruction for volume. Piano, on the other hand, indicates a soft volume level, also without a dynamic change implication. Thus, crescendo is the term specifically associated with the gradual rise in loudness.