Dene Godfrey (my hero of the day) succinctly defines natural by asking 2 simple questions:
1. Does the substance exist in nature?
2. Is the substance extracted from nature without any chemical modification?
Only if the answer to both questions is yes can the substance truly be described as natural.
He goes on to explain the difference between natural and nature-derived. Recommended reading: the entire post "100% Natural? ...Almost 100% Certainly Not"