A quadrilateral is any two dimensional, four-sided shape. Its sides must all be straight. Squares, rectangles and parallelograms are all different kinds of quadrilaterals.

en.wikipedia.org/wiki/Quadrilateral

In Euclidean plane geometry, a quadrilateral is a polygon with four edges (or
sides) and four vertices or corners. Sometimes, the term quadrangle is used, ...

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Quadrilateral just means "four sides" (quad means four, lateral means side). A
Quadrilateral has four-sides, it is 2-dimensional (a flat shape), closed (the lines ...

www.khanacademy.org/math/basic-geo/basic-geometry-shapes/basic-geo-quadrilaterals/v/quadrilateral-overview

Learn the term quadrilateral as well as trapezoid, parallelogram, rectangle,
rhombus, and square.

www.khanacademy.org/math/geometry-home/quadrilaterals-and-polygons

Quadrilaterals only have one side more than triangles, but this opens up an
entire new world with a huge variety of quadrilateral types. Learn about it here.

www.cut-the-knot.org/Curriculum/Geometry/Quadrilaterals.shtml

Classification of Quadrilaterals. Quadrilateral is a geometric shape that consists
of four points (vertices) sequentially joined by straight line segments (sides).

www.geom.uiuc.edu/docs/reference/CRC-formulas/node23.html

5.2 Quadrilaterals. The following formulas give the area of a general quadrilateral
(see Figure 1, left, for the notation). Figure 1: Left: notation for a general ...

www.dummies.com/education/math/geometry/identifying-the-seven-quadrilaterals/

A quadrilateral is a polygon with four sides. There are seven quadrilaterals, some
that are surely familiar to you, and some that may not be so familiar. Check out ...

www.dictionary.com/browse/quadrilateral

Quadrilateral definition, having four sides. See more.

www.math.com/school/subject3/lessons/S3U2L3GL.html

A quadrilateral is a four-sided polygon with four angles. There are many kinds of
quadrilaterals. The five most common types are the parallelogram, the rectangle
...