·adj Having four sides, and consequently four angles; quadrangular.
II. Quadrilateral ·noun A plane figure having four sides, and consequently four angles; a quadrangular figure; any figure formed by four lines.
III. Quadrilateral ·noun An area defended by four fortresses supporting each other; as, the Venetian quadrilateral, comprising Mantua, Peschiera, Verona, and Legnano.