In geometry, a rectangle is a defined as a quadrilateral polygon in which all four angles are right angles.
From this definition, it follows that a rectangle has two pairs of opposite sides of equal length; that is, a rectangle is a parallelogram. A square is a special kind of rectangle where all 4 sides have equal length; that is, a square is both a rectangle and a rhombus. A rectangle that is not a square is colloquially known as an oblong.
Of the two opposite pairs of sides in a rectangle, the length of the longer side is called the length of the rectangle, and the length of the shorter side is called the width. The area of a rectangle is the product of its length and its width; in symbols, A = lw. For example, the area of a rectangle with a length of 5 metres and a width of 4 metres would be 20 square metres, because 5 × 4 = 20. See the picture above.
In calculus, the Riemann integral can be thought of as a limit of sumss of the areas of arbitrarily thin rectangles.