VIX is the ticker symbol for the Chicago Board Options Exchange (CBOE) Volatility Index, a measure of the implied volatility of S&P500 index options. A high value corresponds to a volatile market and therefore means options will be more costly. Often referred to as the fear index or fear gauge, it is a measure of expected volatility over the next 30 days. The VIX was below 30 for most of the time in the 15 years up to 2008. Global financial crisis drove it close to 90 in October 2008.

See also: http://www.cboe.com/DelayedQuote/SimpleQuote.aspx?TICKER=VIX