href="

Phi (upper case Φ or $Φ$ ; lower case φ, φ, $φ$ or $\varphi$ ) is the 21st letter of the Greek alphabet. In modern Greek it is pronounced fee, but a common anglicized pronunciation is fie. In modern Greek, it represents [f], a voiceless labiodental fricative. In Ancient Greek it represented [pʰ], an aspirated voiceless bilabial plosive. In the system of Greek numerals it has a value of 500.

The lower-case letter φ (or often its variant, φ) is used as a symbol for:

The golden ratio 1.618... in mathematics, art, and architecture.
Euler's totient function in number theory. Also called Euler's phi function, φ(n)
The argument of a complex number in mathematics.
The value of a plane angle in physics and mathematics.
Electric potential in physics.
The work function in electronics.
The phase of a wave in signal processing.
In spherical coordinates phi is usually used to represent the angle to the z axis.

The upper-case letter Φ is used as a symbol for;

In engineering, the diameter symbol is often referred to as "phi". This symbol is used to indicate the diameter of a circular section, for example 14 means the diameter of the circle is 14 units.
In structural engineering, Φ is notation for a strength (or resistance) reduction factor, used to account for statistical variabilities in materials and construction methods.
The magnetic flux in physics.
The Cumulative Normal Distribution function in statistics.

Phi (letter)