Poetry Guide: Anacrusis
In poetry, anacrusis is the lead-in syllables that precede the first full measure, while, similarly, in music, it is the note or notes (even a phrase) which precede the first downbeat in a group. The latter sense is synonymous with upbeat and is often called the pickup or the pickup note.
x / x x / x x / x x / Oh, say, can you see, by the dawn's early light. . .