In a comment, John Lawler wrote:

No, there's no term for this, except for zero derivation. Latin didn't even have adjectives. To the Latin grammarians, what we call adjectives were nouns. They had exactly the same inflections and accidence as any other noun, but they lacked grammatical gender. On the other hand, plenty of noun roots (like fili-) also lacked grammatical gender, and could produce either ad lib. There was no reason for them to have a separate category. Since our grammatical terms come from Latin, and they didn't have the concept, they didn't make a term and we didn't borrow it.