Source : Webster's Revised Unabridged Dictionary (1913)

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.

   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.