phi

Phi (Φ or φ) is the 21st letter of the Greek alphabet, and in English it is mostly used as a symbol. Depending on the context, it may be read as "fee" or "fie." You will meet it most often in mathematics, physics, engineering, philosophy, and academic names based on Greek letters.

In mathematics, phi often stands for the golden ratio, an irrational number about equal to 1.6180339887. This number is linked to geometry and to the Fibonacci sequence, because ratios of neighboring Fibonacci numbers get closer and closer to it. In that setting, phi names a specific constant, not just a general variable.

Outside that special use, phi is also a common label for many other ideas. In geometry and physics, it often marks an angle, especially in polar or spherical coordinates. In engineering or science, it may show phase, flux, a potential function, or another quantity chosen by convention. In logic and philosophy, it can simply stand for a statement or placeholder.

You may also see phi in names and organizations, especially in fraternities, sororities, honor societies, and older academic terms built from Greek letters. Because of all these uses, the safest way to understand phi is to read the surrounding context. Sometimes it means the famous golden ratio; just as often, it is simply the Greek letter being used as a symbol.