The VIX, or the CBOE Volatility Index, is a measure of the stock market’s expected volatility over the next 30 days. It is also known as the “fear index” because it tends to rise when there is fear and uncertainty in the market. The VIX is calculated by taking the weighted average of the implied volatilities of a range of S&P 500 index options. The higher the VIX, the more volatile the market is expected to be, and vice versa. It is often used as a gauge of investor sentiment and risk appetite.