Which of the following is a defining characteristic of a trapezoid?

A trapezoid is defined specifically by having at least one pair of parallel sides. This characteristic distinguishes it from other quadrilaterals. In a trapezoid, the parallel sides are often referred to as the "bases," while the non-parallel sides are known as the "legs."

This defining trait means that a trapezoid can have different lengths for its sides and varying angle measures, as long as the condition of parallelism is satisfied. The definition does not require all sides to be equal or congruent, nor does it necessitate that all angles are right angles, which would apply to a specific type of trapezoid known as a rectangle.

Understanding this concept is crucial because it forms the foundation of how trapezoids are categorized and distinguished from other quadrilaterals. Additionally, the other possible characteristics listed, such as having all sides equal or only two sides congruent, do not apply to the definition of a trapezoid.