(1):

(a.) Having four sides, and consequently four angles; quadrangular.

(2):

(n.) A plane figure having four sides, and consequently four angles; a quadrangular figure; any figure formed by four lines.

(3):

(n.) An area defended by four fortresses supporting each other; as, the Venetian quadrilateral, comprising Mantua, Peschiera, Verona, and Legnano.