Autocorrelation
Also called: serial correlation, autocorrelation function, Durbin-Watson
Autocorrelation is a series being correlated with its own past values. In residuals it signals a misspecified model and understated standard errors. In finance it's also a substantive finding: returns show almost none, which supports weak-form efficiency, while squared returns show a great deal, which is volatility clustering and the reason GARCH models exist.
Where this is taught
Definitions are the trailer. These free levels turn Autocorrelation into something you play — one bite-size lesson, with worked examples, a quiz and XP.