The last beat of any measure, usually a weak beat. It is called the "upbeat" because the conductor always directs it with an upward swing of the baton or hand. This anticipates the first beat of the next measure, the "downbeat", always directed by a downward swing of the baton or hand.
SUGGESTED LISTENING EXAMPLES:
Mendelssohn: Symphony No. 4 in A major, Op. 90 (Italian), IV