Element of which a thing is made, determined by its form.
This is philosophy, mainly in the Aristotelian tradition. Materia designates the physical or material substance making up every object in the world, insofar as it is potentially something, while form is what defines and actualizes this matter.
Materia prima is matter deprived of all form, pure potentiality. It cannot exist on its own, and must be combined with forma (shape) to constitute a concrete object.