In music, an anacrusis (also known as a pickup, or fractional pick-up) is a note or sequence of notes, a motif, which precedes the first downbeat in a bar in a musical phrase.
What does an anacrusis do?
Anacrusis, or pickup measure, in piano music. Definition: The anacrusis is a note or series of notes that comes before the first complete measure of a composition; an introductory (and optional) measure that does not hold the number of beats expressed by the time signature.