Four coplanar line segments linked end to end to create a closed figure.
A 4-sided polygon.
Try this Drag the orange dots on each vertex to reshape the quadrilateral.

The quadrilateral is any 4-sided polygon. But it is the parent of a whole family of figures, each with their own distinct names and characteristics. See also Tetragon.

Quadrangle ("four angles") is another name for a quadrilateral. However, it is a word often used for an open space where people gather, say on a school or university campus. In this case the quadrangle (or "quad") is usually a rectangle or square.

 Square All sides equal, all angles 90°.   See Definition of a square Rectangle Opposite sides equal, all angles 90°.   See Definition of a rectangle Parallelogram Opposite sides parallel.   See Definition of a parallelogram Trapezoid Two sides parallel.   See Definition of a trapezoid Rhombus Opposite sides parallel and equal.  See Definition of a rhombus Kite Adjacent pairs of sides equal.  See Definition of a kite