upbeat
The last beat of a bar, which leads into the downbeat of the next bar; also called an anacrusis.
In Depth
An upbeat (or anacrusis) is the note or notes that precede the first strong beat of a phrase. When a conductor raises the baton before bringing it down for beat one, that upward motion is the upbeat — the musical equivalent of drawing breath before speaking.
Many famous melodies begin with an upbeat rather than on the downbeat. The Star-Spangled Banner starts with an upbeat (Oh say...), as does Beethoven's Symphony No. 5 (the three short notes before the long note are upbeats). Starting on an upbeat creates a sense of forward momentum, launching the music toward the first strong beat.
The phrase on the upbeat meaning optimistic comes directly from music — the upbeat leads to the downbeat, so being on the upbeat means anticipating something positive about to happen.