n. A proverbial expression; a proverb.n. An often-used word or phrase.n. One that represents a type, class, or quality: "Polyester got its déclassé reputation in the 1970s after cheap, poorly made double-knit leisure suits became a byword for bad taste” ( Fortune). n. An object of notoriety or interest: The eccentric poet was a byword in literary circles. n. An epithet.