A lowercase Greek letter commonly representing density, correlation coefficients, or radius in polar coordinates.
Examples
Density in physics
\rho = \frac{m}{V} \text{ where } \rho \text{ is density}
Correlation coefficient in statistics
\rho_{xy} = \frac{\text{Cov}(X,Y)}{\sigma_x \sigma_y}
Polar coordinates
x = \rho \cos\theta, \quad y = \rho \sin\theta