\Quad`ri*lat"er*al\, a. [L. quadrilaterus: cf. F.
quadrilat[`e]re, quadrilat['e]ral. See {Quadri-} and
{Lateral}.]
Having four sides, and consequently four angles;
quadrangular.

\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.
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.