Which term describes a persistently repeating rhythmic pattern in the same voice?

The term that describes a persistently repeating rhythmic pattern in the same voice is "ostinato." An ostinato serves as a foundational element in musical compositions, providing a consistent and recognizable backdrop over which melodies and harmonies can evolve. This characteristic of repetition allows it to create a sense of unity and structure within a piece.

In contrast, a riff typically refers to a short, catchy melodic or rhythmic phrase used in various music genres, but it may not always persistently repeat within the same voice. A phrase is a musical segment that conveys a complete idea, often with a beginning and an end, but it does not necessarily imply repetition. A theme, meanwhile, denotes the primary musical idea or subject of a larger work, which may be developed and varied, rather than just being a consistent, repetitive pattern. Thus, "ostinato" aptly captures the essence of a rhythmically repeating pattern within a single voice.