What shape is defined by having two parallel sides?

A shape characterized by having two parallel sides is a trapezoid. This definition is fundamental to the trapezoid, where one of the key properties is that it contains at least one pair of opposite sides that are parallel.

In a trapezoid, the two parallel sides are commonly referred to as the bases, while the other two sides can be either parallel or non-parallel. This unique property distinguishes trapezoids from shapes like rectangles and triangles, where all sides or angles adhere to different sets of rules regarding parallelism and congruency.

For example, a rectangle not only has parallel sides but also has all sides equal in complementary angles (90 degrees), which is not a requirement for a trapezoid. A triangle, on the other hand, does not exhibit any parallel sides at all as it consists of three sides that each connect only at vertices. A circle lacks sides altogether, as it is defined as the set of all points equidistant from a central point. Therefore, the trapezoid is specifically recognized and defined by its property of possessing at least one pair of parallel sides, making it the correct answer.