@ Undo... It is, unfortunately, known as a solo when one instrument takes a more decorated passage and plays it in a progression, be it with or without instrumental backing. The only composers I can think of who would suck all accompaniment out under solos (and only rarely) were late romantic/C20th composers like Shostakovich or slightly earlier, Rakhmaninov. But listen to any of the Brandenberg Concertos (J.S Bach), and look at the scores (if you ever get the chance), clearly marked with violin solo while the choral progressions continue to jump around in a frantic but cohesive fashion. My point being, solo sections can be majorly polyphonic or completely monophonic. Choice of timbre as an element in music doesn't affect structure or form in anyway nowadays (maybe ritornello form contradicts that statement, but then I hold, not many people write like that anymore).