What is defined as a quadrilateral with two pairs of parallel sides?

A quadrilateral with two pairs of parallel sides is defined as a parallelogram. This shape possesses specific characteristics that distinguish it from other quadrilaterals. In a parallelogram, opposite sides are not only parallel but also equal in length, and opposite angles are equal. These properties arise from the definition itself, making parallelograms a fundamental concept in geometry.

While a rectangle and a square are also types of parallelograms (having the additional property of right angles for rectangles and equal side lengths for squares), they require specific conditions beyond just having parallel sides. A trapezoid, on the other hand, is defined as a quadrilateral with at least one pair of parallel sides rather than two pairs, which does not meet the requirements of this question. Thus, the most encompassing answer that fits the definition of a quadrilateral with two pairs of parallel sides is a parallelogram.