The Cboe Volatility Index, an options-based gauge of expected volatility in the S&P 500 widely known by its trading symbol, VIX, jumped Monday morning to its highest level since December.

In recent action, it was trading at 27, after hitting its highest intraday level since Dec. 18. Often referred to as Wall Street's "fear gauge," the VIX often spikes during sharp selloffs. The VIX's long-term average is seen as just below 20.