What is "carroballista"

Carroballista was an ancient, cart-mounted ballista, a type of mobile field artillery. According to the Roman author Vegetius (Epitoma rei militaris II.25), each legion had 55 carroballistae (one per century) which were arrow/bolt-shooter of the cheiroballistra type. Vegetius tells us that each carroballista was carried by mules and operated by one contubernium (i.e. ten private soldiers commanded by one decanus). Surviving representation of a carroballista are from the bas-reliefs of Trajan's Column (Scene XL and Scenes LXV/LXVI) and the Column of Marcus Aurelius.