Volatility Clustering
Also called: vol clustering, persistence of volatility
Volatility clustering is the observation that turbulent days follow turbulent days and calm follows calm. Returns themselves are nearly unpredictable, but their magnitude is strongly autocorrelated — which is the single most robust empirical fact in finance. It's the entire justification for GARCH and EWMA models, and the reason a risk estimate that weights the last month equally with the last decade will always be late to a crisis.
Where this is taught
Definitions are the trailer. These free levels turn Volatility Clustering into something you play — one bite-size lesson, with worked examples, a quiz and XP.