CBOE VIX

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A popular measure of the implied volatility of options of the S&P 500.  High levels of the VIX indicates that investors are fearful of an impending stock market decline, as can be seen in the spike in the index during periods of sharp market declines, like that of the global financial crisis. 

The VIX is often viewed in a contrarian way by technicians.  An extreme spike in the index indicates extreme investor fear which could be overblown, and therefore a good opportunity to buy in.

Synonyms:
VIX