Root meaning: What is in harmony with the nature of a thing or of reality.
Conventional: What we find easy, effortless. What is in accord with the world of (physical) nature.
ACIM: What is of God, since the true nature of everything is God’s nature. Heaven, formless abstraction, is our natural state (see W-pI.161.2:1). On earth, what is truly natural are miracles. The ego, the body and the “natural” world are extremely unnatural. They are habits we taught ourselves with great difficulty over millions of years. As a result, they now seem natural, while what is truly natural seems alien and difficult. See T-16.II.3, W-pI.41.8:1-3.