1. (Science: geometry) 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.
(Science: geometry) Complete quadrilateral, 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.