A Greek letter commonly representing phase angles, scalar fields, or mapping functions in mathematics and physics.
Examples
Golden ratio notation
\phi = \frac{1 + \sqrt{5}}{2} \approx 1.618034
Angle in polar coordinates
r(\phi) = 2\cos(3\phi)
Euler's totient function
\phi(n) = \text{number of integers } k \text{ where } 1 \leq k \leq n \text{ and } \gcd(k,n) = 1