Phi

Phi (uppercase Φ, lowercase φ) is the 21^st letter of the Greek alphabet (unrelated to ordinality but rather to linguistics, its associated with letter f in the Latin alphabet).

It is also used as a symbol in engineering and mathematics:

In engineering, the diameter symbol is often referred to as "phi". This symbol is used to indicate the diameter of a circular section, for example means the diameter of the circle is 14 inches or millimeters, depending on the context.
It is used to represent the Golden mean and the Euler totient function.
In spherical coordinates phi is usually used to represent the angle to the z axis.
The golden mean (1+√5)/2 is often called Φ, φ being 1/Φ.