Concise Bible Dictionary:

That which is according to nature.
1. γένεσις, “origin, birth.” Man beholds his natural face in a glass (James 1:23).
2. κατὰφύσιν, “according to nature.” The Israelites are called the natural branches of the olive tree which God planted on earth (Rom. 11:21,24). φυσικός, “that which belongs to nature” (Rom. 1:26-27; 2 Peter 2:12; Jude 10).
3. φυχικός, from “life, soul.” “The natural man [that is, a man characterized by the natural life of the soul, without the teaching and power of the Holy Spirit] receiveth not the things of the Spirit of God” (1 Cor. 2:14). The body of the Christian is sown “a natural body” (having had natural life through the living soul); it will be raised “a spiritual body” (1 Cor. 15:44-46).