Phi

Phi (uppercase Φ, lowercase φ) is the name of:

• The 21^st letter of the Greek alphabet (unrelated to ordinality but rather to linguistics, it's associated with letter f in the Latin alphabet).

• 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.

• The Greek letter used to represent the so-called 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.

See also: Phi phenomenon.