$R^2 = 0.64$; measures the proportion of variance in $y$ explained by $x$ in a linear model.

$R^2 = -0.64$; measures the strength of the linear relationship.

$R^2 = 0.80$; measures the slope of the regression line.

$R^2 = 0.64$; measures the causal relationship between $x$ and $y$.