Natural law is the concept that some form of law exists naturally and universally, beyond the laws created by governments and societies, and beyond the observable laws of physics and mathematics. It is most closely associated with the concept of objective morality: the belief that some actions will always remain moral or immoral, regardless of the attitudes of individuals, societies or governments to these actions.
The philosophy of natural law is an ancient one, expressed by Plato, Aristotle, and other ancient philosophers. It has also been incorporated into theistic religions such as Christianity, although whether the rules promoted by these faiths can be considered as natural law rather than divine law is rather vague. Popular among conservatives, appeals to natural law may conceal reference to religious authority as the basis for law in secular societies. See Lawler's Libertarian Threat to Human Liberty. However, some atheists also believe in natural law.
The modern concept of human rights was founded on similar principles: an assertion that certain rights are "inalienable" to all human beings. In contrast to natural law, the concept of moral relativism suggests that morality is defined by the attitudes of societies and individuals, and varies according to context. There is certainly some overlap, such as those who support the existence of human rights but believe that other aspects of morality are relative.
A hot topic of debate among libertarians is whether natural laws exist which define an inalienable set of human rights. Murray Rothbard was a notable defender of the concept of natural law. An opposing view, utilitarianism, holds that natural law as such does not exist and human rights are social constructs which serve an agreed-upon human purpose. At the extreme end of the anti- side of the debate are the egoists, who hold to a "might makes right" position.
There was also a Natural Law Party, a multinational political concern essentially created to further the ambitions of the Maharishi Mahesh Yogi, and promote yogic flying as a solution to the world's problems or some such. It has been to all intents and purposes inactive for several years; the United States section more or less closed up shop in 2004 but maintains a web presence.