The ballista (Latin, from Greek ballist?s, from ballein "to throw") is a powerful weapon resembling a giant crossbow, to eject heavy darts singly or in groups. It is also sometimes referred to as a bolt thrower.

Roman ballistae fired large stones, rather than bolts. Ballistae in the Middle Ages launched bolts, and were thought to have inspired the European crossbow, though 5th century Chinese crossbows may have also been a factor in the development of these weapons and bolt-throwing ballista.

A ballista's arms are wood, each supported by being spun into the middle of a large rope. The ropes (one per arm) are the springs of the ballista. The ropes are made of animal sinew. When the bow-arms are pulled back, they twist the ropes. Winches pull the bowstring back.

The ballista was a very accurate weapon, but it had to compromise its accuracy for range. The lightweight bolts could not gain the high momentum of the stones thrown by onagers, trebuchets, or catapults ? these could be as heavy as a 200-300 pound (90-135 kg).

The precise design of ballistae was a triumph of ancient engineering and mathematics. The surviving manuals provide exact measurements and calculations for building ballistae of any size.

The first known appearance was in Syracuse, Italy, around 400 BC.

Its wider development was in the Middle Ages. The catapult was an evolution of the ballista.