Definition notes
trapezoid (US) = trapezium (UK)
In Euclidean geometry, a convex quadrilateral with at least one pair of parallel sides is referred to as a trapezoid in American and Canadian English but as a trapezium in English outside North America.
Some define a trapezoid as a quadrilateral having only one pair of parallel sides (the exclusive definition), thereby excluding parallelograms. Others define a trapezoid as a quadrilateral with at least one pair of parallel sides (the inclusive definition), making the parallelogram a special type of trapezoid.
The latter definition is consistent with its uses in higher mathematics such as calculus.
Source
http://en.wikipedia.org/wiki/Trapezoid
Further Reading