Prima materia, 'first matter', or 'prime matter' is thought to be the starting material required to create the alchemical magnum opus and subsequently a real philosopher's stone. It's thought to be the primitive and formless base of all matter or aether.
It can also pertain oh a philosophical matters as well, but for this post, I'm sticking with alchemy.
The Prima Materia was the foundation for alchemists to create the philosopher's stone; a legendary substance said to be capable of turning base metals such as lead into gold or silver. Like the scientists of today, alchemists sought to learn the secrets of the universe. The concept of Prima Materia is often contributed to Aristotle, but many feel the roots of the idea is found in the philosophy of Anaxagoras, which could explain why most of what I found on this subject was philosophical.
The fact that alchemists over the centuries have given a different names to it, could possible hint as to how elusive this quest is. In researching this topic, I found very little as to give a good definition as to what it really way, apart from the mythical entries.
If Latin doesn't bother you, this link is good for the philosophical interpretation: