**Phi**(Φ φ) is the 21st letter of the Greek alphabet and has a numeric value of 500.

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

- Euler's phi function
`φ`(`n`) - The golden mean.
- In physics and mathematics, the value of an angle.
- The work function.
- The empty set.
- In signal processing, the phase of a sinusoidal signal.

*See also:*