\rho
The Greek letter rho (\rho) is commonly used in mathematics and science to represent various concepts, such as density, correlation coefficient, and resistivity, depending on the context.
Represents density, where m is mass and V is volume.
\rho = \frac{m}{V}
Represents the Pearson correlation coefficient between random variables X and Y.
\rho(X, Y) = \frac{\text{cov}(X, Y)}{\sigma_X \sigma_Y}
Represents electrical resistivity, where R is resistance, L is length, and A is cross-sectional area.
R = \rho \frac{L}{A}