The Collaborative International Dictionary of English v.0.48:

Ballista \Bal*lis"ta\, n.; pl. Ballist[ae]. [L. ballista,
   balista, fr. Gr. ba`llein to throw.]
   An ancient military engine, in the form of a crossbow, used
   for hurling large missiles.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ballista
    n 1: an engine that provided medieval artillery used during
         sieges; a heavy war engine for hurling large stones and
         other missiles [syn: catapult, arbalest, arbalist,
         ballista, bricole, mangonel, onager, trebuchet,
         trebucket]