Definitions
from The American Heritage® Dictionary of the English Language, 4th Edition
 n. Mathematics A polygon having four sides.
 adj. Having four sides.
from Wiktionary, Creative Commons Attribution/ShareAlike License
 n. a polygon having four sides.
 adj. having four sides.
from the GNU version of the Collaborative International Dictionary of English
 adj. Having four sides, and consequently four angles; quadrangular.
 n. A plane figure having four sides, and consequently four angles; a quadrangular figure; any figure formed by four lines.
 n. An area defended by four fortresses supporting each other.
from The Century Dictionary and Cyclopedia
 Having four sides; composed of four lines.
 n. A figure formed of four Straight lines.
 n. Milit., the space inclosed between, and defended by, four fortresses: as, the Bulgarian quadrilateral. The most famous quadrilateral was that in northern Italy, inclosed by the fortresses of Peschiera, Mantua, Verona, and Legnago.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 adj. having four sides
 n. a foursided polygon
Etymologies
Sorry, no etymologies found.
Examples

For instance, the term quadrilateral, or foursided rectilinear figure, is correctly divided into square, oblong, rhombus, rhomboid and trapezium.

In the dead center of the quadrilateral was the sign of the pentagram.

So far as the socalled quadrilateral talks were concerned, these were oneoff talks at officer level between Australia, US, India and Japan.
Transcript  The Hon Stephen Smith MP, Minister for Foreign Affairs

All these stars are of the second order of magnitude (the specific meaning of this expression will be explained in the next chapter), except the last ([delta]) of the quadrilateral, which is of the third order.

Then this rule would to another: opposite angles in the quadrilateral will be the sum of 180 degrees.

A quadrilateral is a polygon with 4 sides, and the term includes square, rectangle, trapezoid, kite, etc.

Mr. Callahan A quadrilateral is a polygon with four sides.

This South Korean loser comedy's love "quadrilateral," in which a Kangwon Province stampmaker (Park Kwangjun) attempts to revenge his cuckolding by a cabdriving womanizer (Jung Boseog), may not be fundamentally any more original than the triangle in Never Forever.

When Lord Roberts had swept past to the north he had brushed aside the flower of the Orange Free State army, who occupied the considerable quadrilateral which is formed by the northeast of that State.

If anybody in this world who didn't have a Ph. D in mathematics wrote this book, they would NEVER have used such a ridiculously pompous word as "quadrilateral" for a square.
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