Introduced on day 3
Covariance
The expected product of two variables' deviations from their means; positive when they tend to move together.
In detail
Correlation is covariance divided by both standard deviations. A simple regression slope equals Cov(X, Y) / Var(X); autocorrelation is the covariance of a series with its own lagged values. Covariance measures linear co-movement only and says nothing about cause.