What do we call a quadrilateral with exactly one pair of parallel sides?

A quadrilateral with exactly one pair of parallel sides is known as a trapezoid. The defining characteristic of a trapezoid is its unique property of having one set of parallel sides while the other two sides are not parallel. This feature distinguishes trapezoids from other types of quadrilaterals, which may have two pairs of parallel sides, like parallelograms, rectangles, and squares.

In the case of a rectangle and a square, both have two pairs of parallel sides, which meets the criteria for being a parallelogram. A parallelogram is a broader category that includes rectangles and squares, but it does not fit the description of having only one pair of parallel sides. Thus, the only term that accurately describes a quadrilateral with precisely one pair of parallel sides is a trapezoid.