What is cousin? What is cousin? A child of one's aunt or uncle. Also called first cousin.A relative descended from a common ancestor, such as a grandparent, by two or more steps in a diverging line.A relative by blood or marriage; a kinsman or kinswoman.A member of a kindred group or country: our Canadian cousins. Something similar in quality or character: "There's no mistaking soca for its distant Jamaican cousin, reggae” ( Michael Saunders). Used as a form of address by a sovereign in addressing another sovereign or a high-ranking member of the nobility.