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` = `l``w`. 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.