The answer depends on which definition of trapezoid (or trapezium, outside of North America) is used.
Some mathematicians define a trapezoid as any four-sided figure with at least one pair of parallel sides. Using that definition, a rhombus is a special type of trapezoid because a rhombus is a figure with two sets of parallel sides.
Others are more strict, using the name trapezoid only for four-sided figures with exactly one pair of parallel sides. By this definition a rhombus is not a trapezoid.