A quadrilateral is a polygon with four sides, four vertices, and four angles summing to 360Ā°.
Quadrilaterals include squares (all sides equal, all angles 90Ā°), rectangles (opposite sides equal, all angles 90Ā°), rhombus (all sides equal, opposite angles equal), parallelogram (opposite sides parallel and equal), trapezoid (one pair of parallel sides), and kite (two pairs of adjacent equal sides). Each type has unique properties and formulas for area and perimeter.
Quadrilaterals were classified by Euclid in his "Elements" (300 BCE). The word "quadrilateral" comes from Latin "quadri" (four) and "latus" (side). Properties of quadrilaterals were studied extensively by Greek and Indian mathematicians. The parallelogram law for vector addition is named after the geometric property of parallelograms.