Rhombus

A rhombus is a parallelogram with all sides equal in length. A rhombus with all angles equal is called a square. The word rhombus comes from the Greek word rhombos, meaning "spinning top". A rhombus is sometimes called a diamond, but not all rhombi are diamond shaped. To find the perimeter of a rhombus, just add up all the sides. Also opposite sides are parallel and opposite angles are equal. Another interesting thing is that the diagonals (dashed lines in second figure) meet in the middle at a right angle.

A rhombus.