The Biblical definition for natural "Revelation" is the Greek word "apokalupsis" which means an uncovering or unveiling. Revelation is the Spirit of God revealing to man that which is otherwise unknowable. He gives to us the knowledge of God:

  1. Cor 2:10 But God has revealed Himself to us through His Spirit. For the Spirit searches all things, yes the deep things of God...

14 But the natural man does not receive the things of the Spirit of God, for they are foolishness to him, nor can he know them, because they are spiritually discerned.