an·a·cru·sis 
(ăn′ə-krsĭs)
(ăn′ə-krsĭs)n.
1. One or more unstressed syllables at the beginning of a line of verse, before the reckoning of the normal meter begins.
2. Music See upbeat.
[New Latin anacrūsis, from Greek anakrousis, beginning of a tune, from anakrouein, to strike up a song : ana-, ana- + krouein, to push.]