Physis (Greek: φύσις) is a Greek theological, philosophical, and scientific term usually

In the Odyssey, Homer uses the word physis once, its earliest known occurrence, referring to the intrinsic way of growth of a particular species of plant. The term occurs very early in Greek philosophy, as early as the pre-Socratic philosophers, and in a number of meanings; generally, these match rather well the current senses in which the English word nature is used. In the Sophist tradition, the term stood in opposition to nomos (νόμος), "law" or "custom," in the debate on which parts of human existence are natural, and which due to convention. This debate brought about early statements of cultural relativism, most notably by Herodotus. Since Aristotle, the physical (the subject matter of physics, properly τὰ φυσικά "natural things") has often been contrasted with metaphysical (the subject of metaphysics).

The etymology of the word "physical" shows its use as a synonym for "natural" in about the mid-15th century. In medicine the element -physis occurs in such compounds as symphysis, epiphysis, and a few others, in the sense of a growing. The physis also refers to the "growth plate," or site of growth at the end of long bones.