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.