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GARCH
In plain language
Markets are never equally volatile: after a crash, the following days and weeks tend to be turbulent too; in calm markets, calm persists. GARCH captures this 'memory effect' in volatility and generates more realistic simulated scenarios than models assuming constant volatility.
Technical definition
Generalized AutoRegressive Conditional Heteroskedasticity: an econometric model in which volatility is not constant but clusters over time. During turbulent periods volatility tends to remain high; during calm periods it compresses.