The Volatility Index (VIX), created by the Chicago Board Options Exchange (CBOE), measures the market's expectation of volatility in the S&P 500 over the next 30 days, derived from options prices. It's widely known as the 'fear gauge' or 'fear index' — when the VIX is high (above 30), markets expect significant turbulence; when it's low (below 20), markets expect relative calm. The VIX typically spikes during crises — it hit 80+ during the 2008 financial crisis and spiked sharply during the COVID-19 selloff. It doesn't predict market direction, only the expected magnitude of moves.