In Euclidean geometry, a parallelogram is a (non self-intersecting) quadrilateral with two pairs of parallel sides. The opposite or facing sides of a parallelogram are of equal length and the opposite angles of a parallelogram are of equal measure. The congruence of opposite sides and opposite angles is a direct consequence of the Euclidean parallel postulate and neither condition can be proven without appealing to the Euclidean parallel postulate or one of its equivalent formulations. The three-dimensional counterpart of a parallelogram is a parallelepiped. A quadrilateral with one pair of parallel sides is a trapezoid in American English or a trapezium in British English.
The etymology (in Greek παραλληλ-όγραμμον, a shape "of parallel lines") reflects the definition.
*Rhomboid – A quadrilateral whose opposite sides are parallel and adjacent sides are unequal, and whose angles are not right angles〔 http://www.cimt.plymouth.ac.uk/resources/topics/art002.pdf〕
*Rectangle – A parallelogram with four angles of equal size
*Rhombus – A parallelogram with four sides of equal length.
*Square – A parallelogram with four sides of equal length and angles of equal size (right angles).
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』