Pearson correlation coefficient

Definition

The correlation coefficient is defined as

where $C_{XY}$ is the covariance of $X$ and $Y$ and $\sigma_X$, $\sigma_Y$ are the standard deviations of $X$ and $Y$ respectively (whereas $\sigma_X^2$, $\sigma_Y^2$ would be the variances).

References

1. https://en.wikipedia.org/wiki/Pearson_correlation_coefficient