1. a formal statement describing a regularity (e.g., of nature) to which no exceptions are known or anticipated. See causal law; natural law.

2. in science, mathematics, philosophy, and the social sciences, a theory that is widely accepted as correct and that has no significant rivals in accounting for the facts within its domain.

3. a formally established rule regulating the conduct of people within a particular jurisdiction that must be obeyed to avoid legal sanctions.