Ziran, literally "self-so", or "so of itself", is the Daoist term for spontaneity and naturalness: the condition of a thing being what it is without external compulsion. It is not "nature" in the sense of the physical world, though modern Chinese uses the same word for that; in the classical texts it names a manner of occurring rather than a domain of things.
Its key statement is in the twenty-fifth chapter of the Daodejing, in a chain that ends the text's account of what takes its model from what: the human takes earth as its model, earth takes heaven, heaven takes the Dao, and the Dao takes ziran as its model. The last term breaks the sequence, because there is nothing beyond the Dao for it to imitate. The line is generally read as saying that the Dao follows nothing but its own being-so, and the chapter has attracted commentary for exactly that reason.
Ziran is closely paired with wu wei, action without forcing. Wu wei describes what the sage does; ziran describes both what results and what everything else is already doing when not interfered with. The political application is direct and is stated as such. The best ruler, the Daodejing says, is one whose existence is barely noticed, and when the work is finished the people say that they did it themselves. Chapter 57 makes the argument explicitly: the more prohibitions there are, the poorer the people become; the more laws are promulgated, the more thieves there are; and so the sage acts without contrivance and the people transform of themselves.
The concept became technically important in the third-century Xuanxue, or "dark learning", movement, where Wang Bi and Guo Xiang made it central and argued over its relation to mingjiao, the teaching of names and social convention. Whether the natural and the conventional are opposed, as some of the period's more deliberately unconventional figures held in their conduct, or ultimately compatible, which was Guo Xiang's considered position, was the live question of the age.
Ziran also shaped Chinese aesthetics profoundly and durably. The highest praise in classical criticism of painting, poetry and calligraphy is that a work appears spontaneous and unlaboured, and the corresponding fault is contrivance, an evaluative standard that presupposes the Daoist concept and that went on operating long after the philosophical debates that produced it had moved elsewhere.