What does the musical term anacrusis mean?

In music, an anacrusis is a note or chord that precedes a downbeat or the main pulse in a musical piece, and is therefore said to be "up beat". An anacrusis can be the last in the previous bar or at the beginning of a new bar/meter. These unaccented introductory notes or beats before a downbeat are not usually notated in the time signature, but they play an important role in creating rhythmic momentum and anticipation in a musical composition.