1. Geom. A plane figure having four sides, and consequently four angles; a quadrangular figure; any figure formed by four lines.
2. An area defended by four fortresses supporting each other; as, the Venetian quadrilateral, comprising Mantua, Peschiera, Verona, and Legnano.
Complete quadrilateral Geom., the figure made up of the six straight lines that can be drawn through four points, A, B, C, I, the lines being supposed to be produced indefinitely.