Quadrilateral
A quadrilateral is a two-dimensional, closed shape with four straight sides, four vertices, and four interior angles. The sum of the interior angles in any planar quadrilateral is always 360 degrees.
Example
A rectangle is a quadrilateral where all four interior angles are exactly $90^\circ$. If the length is $l$ and the width is $w$, its area is calculated as $A = l \times w$.