VIX = Volatility Index

CBOE Volatility Index, VIX, is a popular measure of the stock market's expectation of volatility based on S&P 500 index options. 

The VIX Index is a calculation designed to produce a measure of constant, 30-day expected volatility of the U.S. stock market, derived from real-time, mid-quote prices of S&P 500 Index call and put options.