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From The Collaborative International Dictionary of English v.0.48:
Quadrilateral \Quad`ri*lat"er*al\, n. 1. (Geom.) A plane figure having four sides, and consequently four angles; a quadrangular figure; any figure formed by four lines. [1913 Webster] 2. An area defended by four fortresses supporting each other; as, the Venetian quadrilateral, comprising Mantua, Peschiera, Verona, and Legnano. [1913 Webster] 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. [1913 Webster]