law
noun:a rule or system of rules established by a government or other authority to regulate behavior and maintain order
It is against the law to drive above the speed limit.
She decided to pursue a career in law and became a successful attorney.
Newton's law of universal gravitation explains the attraction between masses.
noun:a customary practice or convention that guides behavior within a particular group or profession
In journalism, protecting your sources is an unwritten law of the profession.