A variant form of the Greek capital letter Phi commonly used as a function symbol in complex analysis and topology.
Examples
Potential function in physics
\varPhi(r) = -\frac{GM}{r}
Cumulative distribution function in statistics
\varPhi(x) = \frac{1}{\sqrt{2\pi}} \int_{-\infty}^x e^{-t^2/2} dt
Euler's totient function notation (alternate)
\varPhi(n) = n\prod_{p|n}\left(1-\frac{1}{p}\right)