What musical term is used to describe the "loudness" or "softness" of a piece?

The term that refers to the "loudness" or "softness" of a piece of music is dynamics. Dynamics encompass the variations in volume throughout a composition, indicating how loudly or softly music should be played or sung. This aspect of music is critical for conveying emotion, intensity, and contrast within a piece. Musicians use specific dynamic markings, such as "piano" for soft and "forte" for loud, to guide their performances and communicate the intended expressive qualities of the music.

In contrast, rhythm pertains to the pattern of sounds and silences in music and how they unfold over time; melody refers to a sequence of notes that are perceived as a single entity; and harmony involves the combination of different musical notes played or sung simultaneously. Each of these concepts plays a significant role in music, but they do not directly address the aspect of volume, which is the focus of dynamics.