Una corda - Soft pedal.

Unison - Singing or playing the same notes by all singers or players, either at exactly the same pitch or in a different octave.

Un peu - A little. Used with other words, e.g. un peu piano.

Un poco - A little.

Upbeat - One or more notes occurring before the first bar line, as necessitated by the text for the purpose of desirable accent. The unaccented beat of a measure.