A quadrilateral with two parallel sides is known as trapezoid.
A square may or may not be a trapezoid, or trapezium. That's because there is a bit of a difference of opinion as to the definition of a trapezoid. One definition of the trapezoid is that is has one pair of parallel sides. The square meets this condition. Another definition of the trapezoid is that is has exactly one pair of parallel sides. The square cannot meet that definition because it has two pairs of parallel sides.
I. Four sides.
II. At least one pair of opposite sides is parallel.
III. Angles between pairs of parallel sides are supplementary.
Where h is the height, and a and b are the lengths of the parallel sides.
= a + b + c + d