Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

ballista
     n : medieval artillery used during sieges; a heavy war engine
         for hurling large stones and other missiles [syn: {catapult},
          {arbalest}, {arbalist}, {bricole}, {mangonel}, {onager},
          {trebuchet}, {trebucket}]
     [also: {ballistae} (pl)]