mid-14c., "the customary and unwritten laws of England as embodied in commentaries and old cases" (see common (adj.)), as opposed to statute law. Phrase common law marriage is attested from 1909.
Law developed in the course of time from the rulings of judges, as opposed to law embodied in statutes passed by legislatures (statutory law) or law embodied in a written constitution (constitutional law). (See stare decisis.)
Note: The importance of common law is particularly stressed in the legal system of Britain, on which the legal system of the United States is based.