anacrusis

from The Collaborative International Dictionary of English v.0.48

Anacrusis \An`a*cru"sis\, n. [Gr. ?, fr. ? to push up or back; ?
   + ? to strike.] (Pros.)
   A prefix of one or two unaccented syllables to a verse
   properly beginning with an accented syllable.
   [1913 Webster]