Meaning

noun a principle or condition that customarily governs behavior

noun something regarded as a normative example

noun prescribed guide for conduct or action

originFrom Middle English reule, rewle, rule, borrowed from Old French riule, reule, from Latin regula (“straight stick, bar, ruler, pattern”), from regō (“to keep straight, direct, govern, rule”), ultimately from Proto-Indo-European *h₃réǵeti (“to straighten; right”), from the root *h₃reǵ-; see regent. Doublet of rail, regal, regula, and rigol.

Etymology from Wiktionary, CC BY-SA 3.0