Phi (upper case Φ or $\backslash Phi$; lower case φ, , $\backslash phi$ or $\backslash varphi$) is the 21st letter of the Greek alphabet. In modern Greek, it represents a voiceless labiodental fricative. In Ancient Greek it represented [, an aspirated voiceless bilabial plosive. In the system of Greek numerals it has a value of 500.

The lower-case letter φ (or often its variant, $\backslash varphi$) is used as a symbol for:

• The golden ratio 1.618... in mathematics, art, and architecture.
• Euler's totient function in number theory. Also called Euler's phi function, φ(n)
• The argument of a complex number in mathematics.
• The value of a plane angle in physics and mathematics.
• Electric potential in physics.
• The work function in electronics.
• The phase of a wave in signal processing.
• In spherical coordinates phi is usually used to represent the angle to the z axis.

• Any function in mathematics.

The upper-case letter Φ is used as a symbol for; is often referred to as "phi". This symbol is used to indicate the diameter of a circular section, for example Boundy diameter symbol.png14 means the diameter of the circle is 14 units.

See also phi phenomenon.

Greek letters

Φ | Phi (lizherenn) | Фи | Fi | Phi | Phi | Φι | Φ | Phi | Phi | Fi | Φ | Phi (lettera) | פי | Phi (letter) | Φ | Phi | Phi | Phi | Fi | Φ | Fii | Fi | Φ