For law within legal systems see Law.
The term "law" is often used to refer to universal principles that describe the fundamental nature of something, to universal properties and relationships between things, or to descriptions that purport to explain these principles and relationships. For example, "physical law"s, or "scientific laws" attempt to describe the fundamental nature of the universe itself. Laws of mathematics and logic describe the nature of rational thought and inference. Laws of economics describe the nature of human behavior and interaction.
Within most fields of study, and in science in particular, the elevation of some principle of that field to the status of "law" usually takes place after a very long time during which the principle is used and tested and verified. Though in some fields of study such laws are simply postulated as a foundation and assumed. Mathematical laws are somewhere in between: they are often arbitrary and unproven in themselves, but they are sometimes judged by how useful they are in making predictions about the real world.
Finally, the term is sometimes applied to less rigorous ideas that may be interesting observations or relationships, practical or ethical guidelines (also called rules of thumb), and even humorous parodies of such laws.
Examples of scientific laws include Boyle's law of gases, conservation laws, Ohm's law, and others. Laws of other fields of study include Occam's razor as a principle of philosophy and the Pareto principle of economics. Examples of observed phenomena often described as laws include the Titius-Bode law of planetary positions, Zipf's law of linguistics, Moore's law of technological growth. Other laws are pragmatic and observational, such as the law of unintended consequences.