Phi

Greek alphabet
Α α Alpha	Β β Beta
Γ γ Gamma	Δ δ Delta
Ε ε Epsilon	Ϝ ϝ Digamma
Ζ ζ Zeta	Η η Eta
Θ θ Theta	Ι ι Iota
Κ κ Kappa	Λ λ Lambda
Μ μ Mu	Ν ν Nu
Ξ ξ Xi	Ο ο Omicron
Π π Pi	Ϻ ϻ San
Ϙ ϙ Qoppa	Ρ ρ Rho
Σ σ Sigma	Τ τ Tau
Υ υ Upsilon	Φ φ Phi
Χ χ Chi	Ψ ψ Psi
Ω ω Omega	Ϡ ϡ Sampi

Phi (upper case Φ, lower case φ), pronounced fee, is the 21st letter of the Greek alphabet. In the system of Greek numerals it has a value of 500.

The lower-case letter φ is used as a symbol for:

• Euler's totient function in number theory.
• The argument of a complex number in mathematics.
• The golden ratio in mathematics, art, and architecture.
• The value of a plane angle in physics and mathematics.
• 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.
• 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 14 means the diameter of the circle is 14 inches or millimeters, depending on the context.

The upper-case letter Φ is used as a symbol for:

• The magnetic flux in physics.

See also Phi phenomenon.