Phi (letter)

Greek letters
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;
Greek alphabet

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 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.