n (context|geometry|British) A four-sided polygon with two sides parallel; a trapezoid (modern sense)
n (context|geometry|US) A four-sided polygon with no parallel sides and no sides equal; a simple convex irregular quadrilateral.
n A region on the ventral side of the brain, either just back of the pons Varolii, or, as in man, covered by the posterior extension of its transverse fibers.