Physical laws are the set of rules and principles governing physical phenomena in the universe. There are several synonyms that can be used for the term "physical law" such as natural law, fundamental law, principle, rule, theory, and axiom. The term "natural law" refers to the physical laws that govern the natural world and are considered to be unchanging and universal. "Fundamental law" refers to the basic laws that form the foundation of all physical phenomena. "Principle" refers to a general rule that is commonly accepted and applied in a particular field. "Theory" refers to a scientifically accepted explanation for a particular phenomenon. And finally, "axiom" refers to a self-evident truth that is accepted without proof.