The mos maiorum is the unwritten code from which the ancient Romans derived their social norms. It is the core concept of Roman traditionalism, distinguished from but in dynamic complement to written law. The mos maiorum was collectively the time-honoured principles, behavioural models, and social practices that affected private, political, and military life in ancient Rome.

Roman law is the legal system of ancient Rome, including the legal developments spanning over a thousand years of jurisprudence, from the Twelve Tables, to the Corpus Juris Civilis ordered by Eastern Roman emperor Justinian I. Roman law forms the basic framework for civil law, the most widely used legal system today, and the terms are sometimes used synonymously. The historical importance of Roman law is reflected by the continued use of Latin legal terminology in many legal systems influenced by it, including common law.