The golden ratio (Latin: Sectio Aurea) is a mathematical relationship where a line segment or a figure is divided so that the ratio of the smaller part to the larger part is equal to the ratio of the larger part to the whole. 

The Pythagoreans are thought to have invented the golden section in the 5th century BCE. The idea probably originated from a regular pentagon. When diagonals are drawn on the pentagon, a new regular pentagon is created at their intersection. The drawing can be continued indefinitely. The diagonals of the pentagons divide each other in proportion to the golden ratio.

The golden ratio occurs in many different contexts. The ratio of two consecutive numbers in the Fibonacci number sequence approaches the ratio of the golden ratio. The golden ratio has been widely used in the visual arts. At one time, mystical and divine features were also associated with the golden ratio.

The golden ratio is a method that facilitates image composition and can be applied to drawings, paintings, and light and video photography. If, for example, an important figure is placed in the middle of a painting, the painting easily becomes clumsy and lifeless-looking. Instead, the picture becomes more interesting and balanced if the character is placed according to the golden ratio. Even Renaissance artists used the golden ratio to bring aesthetic harmony to their works of art. Artists such as Leonardo da Vinci carefully studied the proportions of man and nature in general. 

The earliest application of the golden section in architecture is the Parthenon temple in Athens, which dates back to ancient times. A newer example is the UN headquarters in New York, which was completed in 1952.