Which term describes a melodic idea that is used and varied in a composition?

A theme is a fundamental concept in music that refers to a melodic idea that serves as the foundation for a composition. It is often introduced at the beginning and can undergo various transformations and developments throughout the piece. This might include alterations in rhythm, harmony, dynamics, and instrumentation, providing depth and complexity to the music.

In contrast, a motif is typically a shorter musical idea or fragment that can contribute to the overall theme, but does not encompass the full melodic structure. An ostinato is a repeated musical phrase or rhythm that provides a framework within a piece, while a counter melody functions as a secondary melody, played alongside a primary melody. Both motifs, ostinati, and countermelodies may play essential roles in compositions but do not fully encapsulate the concept of a theme in the same comprehensive and varied manner.