A variant form of the Greek letter rho commonly used as a secondary density symbol in physics and mathematics.
Examples
Density function in physics
\varrho(x,t) = \frac{m}{V(t)}
Correlation coefficient in statistics
\varrho_{X,Y} = \frac{\text{Cov}(X,Y)}{\sigma_X \sigma_Y}
Resistivity in electrical engineering
\varrho = \frac{RA}{l}