A ballista is a siege weapon used to launch projectiles.
The technical meaning of a ballista is a crossbow powered by torsion as opposed to a tension. What this means is that a ballista is based on the rotational movement of a spring, as opposed to the release of a tight string. Ballistae ranged from small, handheld crossbows to larger catapults which could break down walls.
In Mossflower, Timballisto uses a ballista to break down the walls of Kotir, which had already been weakened by flooding. However, in the book, the ballista is described as consisting of counterweights and a throwing arm, which more describes a trebuchet.
Yet in Mossflower, the ballista throws rocks and is refered to as a catapult. Also, it has a release lever (which Dinny enjoyed pulling). It would seem likely, based on the description of the machine, that the ballista was actually a trebuchet. As for the name, there appear to be two possibilities. Either "ballista" is the Redwaller's name for catapults, just like Dibbun is the equivalent of "Abbeybabe." Alternatively, it is possible that Brian Jacques made an error in his description.
In The Sable Quean, Gurjee oversaw the construction of a siege catapult in the cellar by Foremole Darbee's mole crew. After Ambrevina Rockflash arrived at Redwall Abbey, she made adjustments to the device, which was described as a ballista. She had experience constructing the weapon, which she had used previously to sink corsair ships with her family on the Eastern Shores. The weapon later proved to be extremely effective in battle, but was destroyed shortly after.