What term indicates that music should be played very softly?

The term that indicates music should be played very softly is "pianissimo." This Italian term is commonly used in musical notation and is often abbreviated as "pp." It instructs performers to produce sound at a very low dynamic level, creating a sense of delicacy and quietness in the performance.

In contrast, the other terms relate to different levels of volume. "Forte" indicates that music should be played loudly, while "piano" means soft, but not to the extreme that "pianissimo" does. "Mezzo-piano" refers to a moderately soft dynamic, which is also not as quiet as "pianissimo." This distinction among these dynamic markings allows composers to convey specific emotional and dramatic intentions within their music.