**Phi (letter)**

From Wikipedia, the free encyclopedia.

**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 totient function in number theory.
- The argument of a complex number in mathematics.
- The golden ratio in mathematics and art.
- The value of a plane angle in physics and mathematics.
- The work function in electronics.
- The phase of a wave in signal processing.

- The magnetic flux in physics.