phi; Φ, φ +

(Greek: the twenty-first letter of the Greek alphabet; Φ, φ)

phi
The twenty-first letter of the Greek alphabet (Φ,φ), represented in the English alphabet as ph and f.

In the system of Greek numerals it has a value of 500 (φʹ) or 500,000 (͵φ).

phi grade scale, phi scale
In geology, a logarithmic scale in which the diameter value of the Wentworth scale is replaced by the negative logarithm to the base two of the particle diameter (in millimeters), so that conventional statistical methods can be applied to sedimentary data.

You can see all of the letters of the Greek Alphabet on this page.