An axiom is a universally accepted rule or principle or a self-evident truth: "There can be only one straight line between two points." "It is an axiom of economics that supply and demand are closely related." An adage is a statement given credit by long usage and general acceptance: "It never rains but it pours." "It is always darkest before the dawn." A proverb is a short saying, usually of unknown origin, that expresses a useful thought or commonplace truth: "A rolling stone gathers no moss." "A stitch in time saves nine." The general word saying embraces the meanings of these terms and also those of aphorism, maxim, apothegm, epigram, and motto.
Dictionary of problem words and expressions. Harry Shaw. 1975.