A closely watched U.S. stock market index has slipped briefly into the dreaded “bear market” territory, the negative point at which it had fallen more than 20 percent from its recent peak last April.

The world's stock markets mostly plunged again Tuesday on continued worries about a Greek debt default and a slowing world economy.

As investors sold their securities, Wall Street's broad-based Standard & Poor's index of 500 large companies slid more than 2 percent. That pushed the index to a reading below 1,075, more than 20 percent below its recent high on April 29th of more than 1,363.

But as Asian and European markets closed sharply lower, the S&P 500 regained some strength in mid-day trading, moving up and hovering close to its Monday finishing point.

In the U.S., stock advances for months at a time are deemed to be part of a “bull market,” while a sharply falling market is called a “bear market.” Financial analysts often use the terms when there is a 20 percent swing in market value, either up or down, although there is no official percentage attached to the terms.

Financial historians have cited various origins for the terms. Some think a bull or bear market can be attributed to actions of the two animals, with spirited and burly bulls thrusting their horns into the air, while bears at times can be sluggish and swipe down with their paws.

In the U.S., bears have been linked to financial transactions at least since the early 18th century. Frontier traders speculated on the price of bearskins they sold, even though they had yet to receive them from trappers. The traders were betting the actual price of the skins would be lower than what they prospectively sold them for, and thus they would make a profit.

As it turned out, the middlemen in the bearskin trade came to be known as “bears.” Now, three centuries later, Wall Street still defines its falling fortunes as a bear market.