trapezoid, closed plane figure bounded by four line segments, or sides, two of which are parallel and two of which are nonparallel. The parallel sides of a trapezoid are called bases and the nonparallel sides legs; in an isosceles trapezoid the legs are of equal length. The median of a trapezoid is the line segment connecting the midpoints of the legs; it is parallel to the bases and equal to half the sum of their lengths. The altitude of a trapezoid is the perpendicular distance between the bases. The area of a trapezoid is equal to half the product of the altitude and the sum of the bases, i.e., to the product of the altitude and the median.