Ballista

from 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}]

from 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]

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