Final answer:
The shape described, having two equal opposite parallel sides but not being a rectangle, is likely a parallelogram.
Step-by-step explanation:
If you have two equal opposite parallel sides but are not a rectangle, the shape you are most likely describing is a parallelogram. A parallelogram is a four-sided shape where the opposite sides are parallel and equal in length, but the angles are not necessarily 90 degrees, unlike in a rectangle. Other possible shapes with these characteristics could include a rhombus or a trapezoid, but in the case of a rhombus, all sides are equal, and a trapezoid has only one pair of parallel sides.