Of God and His Creatures

A body, according to St Thomas, is made up of a potential subject, called 'matter,' connaturally extended in space; and further of an actuating principle of energy, called 'substantial form,' which is so united to the potential subject, or 'matter,' that the latter thereby becomes an individual body within a definite species, deriving its power of action from the 'substantial form,' or principle of energy.

Of God and His Creatures: 1.23