When I was growing up, I learned the "at least one pair of parallel sides" definition of a trapezoid. I have always stuck by this definition. However, this is a matter of debate throughout the math world - the inclusionary definition and the exclusionary definition. I say it is a case of "all squares are rectangles, but not all rectangles are squares" type logic. At least one pair of parallel sides works for all parallelograms when looking at formulas for area. The formula for the area of a trapezoid works for all parallelgrams as well.