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]