A trapezoid has which defining feature?

A trapezoid is defined by the fact that it has at least one pair of opposite sides that are parallel. This distinguishing characteristic is what sets trapezoids apart from other quadrilaterals. In the case of the trapezoid, the two sides that are parallel are often referred to as the bases, while the other two sides are not necessarily parallel and can be of any length.

Thus, when considering the options, the definition that states "two opposite sides are parallel, the other two sides not" accurately describes this essential feature of a trapezoid. It's important to understand that while some trapezoids can have additional properties, such as angles being equal or other sides being parallel (as seen in specific types like isosceles trapezoids), the fundamental definition requires only a single pair of parallel sides.