passage work

noun Music.

1. writing that is often extraneous to the thematic material of a work and is typically of a virtuosic or decorative character: passagework consisting of scales, arpeggios, trills, and double octaves.
2. the performance of such writing: The pianist’s passagework is brilliantly clear and smooth.

noun

1. music scales, runs, etc, in a piece of music which have no structural significance but provide an opportunity for virtuoso display