A trapezoid has four sides, so it's a quadrilateral. One of the most distinctive features of trapezoids is that they have exactly one pair of parallel sides. trapezoid

The sides of a trapezoid that are parallel are called bases, while those that are not parallel are called legs.  

base and leg of a trapezoid

The midsegment of a trapezoid bisects both its legs.

midsegment of trapezoid