The S&P 500 sunk into bear market territory on December 24, 2018. A “bear market” is when stocks see a 20 percent decline or more from a recent high — but they’re also marked by overall pessimism on Wall Street. CNBC's Jeff Cox breaks down how to maneuver a bear market investing environment.

Since World War II, bear markets have lasted 13 months on average, and stock markets tend to lose 30.4 percent of their value. During those conditions it usually takes stocks an average 22 months to recover, according to analysis from Goldman Sachs and CNBC.

It's helpful to know what a "bear market" is, because based on history it looks like we could be here for a while.

The term on Wall Street is synonymous with serious, long-lasting declines in stock markets. In numeric terms, a bear market is a 20 percent or more drop from a recent peak.

The S&P 500 hit that milestone on Monday December 24, dropping 20 percent from its 52-week high. Markets have stumbled through what is usually one of their best months of the year, with indexes on track for their worst December performances since 1931, during the Great Depression.

Aside from a percentage drop, there are other, more emotional ways to measure a bear market.

Pessimism tends to prevail. When good news isn't enough to hold off sellers and despite solid economic conditions, markets continue to tank — that's a bear market. The glass-half-full scenario is often overlooked, and any positive news seems to be forgotten by the close of trading.

In December, oversold markets struggled to make a comeback, suggesting that investors are worried about something bigger. Still, economic fundamentals are not giving red flags of a recession, which is usually a necessary condition for a full-fledged bear market.
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How To Invest In A Bear Market