Un peu: (Fr.) A little
Unison: to sing or play the same notes and pitch; sometimes players are said to be playing in unison even when they are playing the same notes an octave or several octaves apart.
Upper partials: Overtones.
Upbeat: 1. Upward stroke made by a conductor indicating the beat that leads into a new measure. 2. The last and unaccented beat of a measure. 3. a song that's positive-sounding, fast.