What is the term for a musical theme in the bass that is repeated multiple times while the upper parts vary?

The term that describes a musical theme in the bass that is repeated multiple times while the upper parts vary is known as Ground Bass. This technique is characterized by a continuous bass line that remains consistent throughout the piece, providing a stable harmonic foundation over which melodies and harmonies in the higher parts can change and evolve.

Ground bass can be found in various musical forms and styles, allowing composers to create a sense of structure and cohesiveness while simultaneously exploring different melodic ideas above the repeated bass line. This approach has been famously employed in works such as Pachelbel's "Canon in D" and Purcell's "Dido's Lament," where the bass line provides a persistent thematic element against more elaborate upper voices, creating an intriguing contrast and depth in the music.

In contrast, the other options describe different musical concepts. For example, a counter melody refers to a secondary melody that complements the main melody. A melodic sequence involves repeating a motif or melody at different pitches. An arpeggio consists of the notes of a chord played in succession rather than simultaneously. Each of these terms illustrates different aspects of musical composition but does not capture the specific concept of a repeated bass theme as is found in a ground bass.