The CBOE Volatility Index, a real-time measure of expected 30-day volatility in the S&P 500 index derived from the prices of near-term SPX options. Often called the market's 'fear gauge,' the VIX tends to rise during periods of market uncertainty and decline when markets are calm and trending higher. A VIX reading below 15 generally indicates complacency, while readings above 30 suggest elevated fear and potential for large market swings.
Example
“The VIX spiked from 14 to 35 in a single session after the surprise geopolitical event, indicating that options traders were pricing in sharply higher expected volatility.”