Scholars have traced the origins of the word "bear" to the 18th century fur traders, who sometimes sold the bear skins they planned to trap before they had even captured the bears. Thus, they hoped prices would fall after they sold. And the term "bull" has been said to derive from the old blood sport that pitted bears against bulls in fights to the death.

Other market historians point to the illustrations of a bull and bear as describing market trends: The full has its horns pointed upward; the bear claws point downward.

While there is little agreement on the origins or the symbols, there is much greater accord when it comes to describing what constitutes a bull or bear market.  While a bull market is often described as an "extended period" of rising stock prices, a bear market is typically defined as a price decline of 20 percent or more over at least a two-month period.  While that measure doesn’t tell you how much farther the market will fall, or how long the bear market will last, it does at least define expectations that prices will fall further.

According to market historian Jim Stack, the median duration of a bear market is 15 months—from peak to bottom. The historic bear markets of 1929 and 1938 lasted 33 and 42 months, respectively.  The current bear market, which started in 2007, has lasted 17 months.

Just as bear markets differ in duration, they also differ in depth of decline.  Stack’s research shows the average bear market decline (excluding 1929) is 33.5 percent.  But the 1973-74 and the 2000-01 bear markets each saw broad market indexes decline nearly 50 percent. The current bear market (based on the S&P 500 index) peaked October 9, 2007, and fell 57 percent to its recent bottom on March 9.

Each bull or bear market has its own characteristics. And that is what makes it so hard to define them, except in hindsight. Even declining markets may contain sharp rallies upward. That generates the debate over whether a sudden upturn marks the end of the bear and the start of a new bull market. Or whether it is just "a rally in a bear market." But Stack, a respected market technician, says we've already seen the bottom and are at the start of a new bull market.

Stack points out that his technical indicators such as the "advance/decline line" as well as measures of "relative strength" and even popular sentiment indicate the market has already seen its lows.