Which term describes a quadrilateral with two parallel lines?

A quadrilateral with two parallel sides is specifically known as a trapezoid. This definition is fundamental in geometry, and it accurately characterizes the shape based on the presence of one pair of parallel sides. Trapezoids can have various properties depending on their specific type; for instance, if the non-parallel sides are equal in length, the trapezoid is referred to as an isosceles trapezoid.

In contrast, a rectangle, rhombus, and square all possess additional properties that define them further. A rectangle has two pairs of parallel sides and right angles, a rhombus has all sides equal with opposite angles equal, and a square combines the properties of both a rectangle and a rhombus by having all sides equal and right angles. While it is true that both rectangles and squares contain parallel sides, the term trapezoid is the most accurate descriptor of a quadrilateral with exactly one pair of parallel sides, fulfilling the condition presented in the question.