A capital Greek letter Phi commonly used to represent scalar fields, potential functions, or cumulative distribution functions in mathematics.
Examples
Standard normal cumulative distribution function
P(X \leq x) = \Phi(x) = \int_{-\infty}^x \frac{1}{\sqrt{2\pi}} e^{-t^2/2} dt
Magnetic flux in physics
\Phi_B = \iint_S \vec{B} \cdot d\vec{A}
Euler's totient function (alternate notation)
\Phi(n) = n\prod_{p|n} \left(1-\frac{1}{p}\right)