Parallelogram
A parallelogram is a four-sided plane figure that has two sets of opposite parallel sides.
Every parallelogram is a polygon, and more specifically a quadrilateral.
Special cases of a parallelogram are the rhombus, in which all four sides are of equal length, the rectangle, in which the two sets of opposing, parallel sides are perpendicular to each other, and the square, in which all four sides are of equal length and the two sets of opposing, parallel sides are perpendicular to each other.
The three dimensional equivalent of a parallelogram is a parallelepiped.
In any parallelogram, the diagonals bisect each other, i.e, they cut each other in half.
The parallelogram law distinguishes Hilbert spaces from other Banach spaces.