Hayūlā is the philosophical concept of prime matter understood by its receptivity to different bodily forms. It is not itself an ordinary piece of wood, clay, or metal already possessing a determinate shape and kind.
In this scheme a body is not simply a heap of stuff: the relation of matter to form makes an identifiable body. Prime matter is posited to explain what remains as different bodily forms arise.
Critical distinction: an observable physical object is not identical to prime matter proposed as a philosophical abstraction. Whether prime matter is eternal still requires argument rather than being assumed from the model.