By definition, we have to:
In plane geometry, a rectangle is a parallelogram whose four sides are at right angles to each other. Opposite sides have the same length.
There is a proof that a quadrilateral is a rectangle:
1) Its parallel sides are the same.
2) Its two diagonals are the same, and they bisect each other at the common midpoint
3) Any rectangle can be inscribed in a circle, two of whose diameters coincide with the diagonals of the rectangle.
4) If all the angles of a quadrilateral are right angles, then it is a rectangle