Sample correlation matrix ($R_{jk}=S_{jk}/\sqrt{S_{jj}{S}_{kk}}$)

A sample correlation matrix rescales a sample covariance matrix $S$ to unit diagonal, for variables with positive sample variance. Its off-diagonal entries are sample correlation coefficients. Strong correlations between predictors can indicate multicollinearity, but pairwise correlations alone do not exclude higher-order linear dependence.