Final answer:
Diminuendo is the term used to describe the volume of music gradually getting softer. It is a dynamic instruction in written music distinct from terms like forte, piano, and crescendo.
Step-by-step explanation:
The term that indicates the volume gradually getting softer in music is diminuendo. This term is one of several dynamic instructions used in written music to guide musicians on how to play a piece. When a composer wants to indicate that the music should decrease in loudness, they would write diminuendo or its abbreviation, 'dim.', into the score. This is different from crescendo, which signals to gradually increase in volume, or dynamic level indicators such as forte (loud) and piano (soft), which describe the intensity of sound at a certain moment rather than a change over time.