Φ (Phi) is the 21st letter of the Greek alphabet.

In mathematics, it is used to represent a number known as the Golden Mean, which is the ratio of two sides of a rectangle called the Golden Section that has very special properties.

The schematic image on the right side of this site represents how the Golden Section is constructed geometrically. Mathematically, the number Φ is equal to (1 + √5) / 2. It is related to the famous Fibonacci Series of numbers [0, 1, 1, 2, 3, 5, 8, 13, 21, …] and has many wonderful properties that connect it to nature, art, and design.

The 1959 Disney animated short film entitled “Donald in Mathmagic Land,” where Donald Duck gets an education in the beauty of mathematics, is wonderful. Among other things, Donald learns about the Golden Section.
This very beautiful, and much more modern, video, “Nature by Numbers,” illustrates the Golden Section and other examples of math in nature.