Mos maiorum

The mos maiorum ("ancestral custom") 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 (plural mores, cf. English "mores") was collectively the "time-honored principles," behavioral models, and social practices that affected private, political, and military life in ancient Rome.

Source: Wikipedia