An Italian musical term used to describe an effect usually found in Baroque-period music where the composer has a note on every quaver (or comparable rhythm eg. semiquaver, triplet quaver, etc.) so as to give a feeling of perpetual motion (it doesn't have to be the smallest rhythmic value - there is still moto perpetuo even if there are semiquavers, provided that the quavers never stop). Most often, this continues until the piece's final chord.
Used to great effect by masters such as Bach (1685-1750), for example in the first movement of his Brandenburg Concerto no. 4. Also used by Debussy (1862-1918) in the Passepied of Suite Bergamasque.