Which term refers to a small musical unit that serves as the basis of a musical composition?

The term that refers to a small musical unit that serves as the basis of a musical composition is "motive." A motive is typically a short melodic or rhythmic idea that can be developed and transformed throughout a piece of music. It often forms the building block for larger sections of a composition, making it integral to the overall structure and thematic development.

In contrast, a theme is generally a broader melodic idea that is more fully developed than a motive and often serves as a primary subject in larger works. A phrase is a complete musical thought that may consist of several motives or themes and usually spans several measures. Harmony refers to the vertical aspect of music, involving chords and their progression, but does not pertain specifically to a small musical unit. Understanding these distinctions enhances comprehension of how composers construct their works.