Stocks' staggering skid that began less than three weeks ago has pulled Wall Street into what's known as a bear market.

The collapse fueled by uncertainty surrounding the coronavirus has officially ended the bull market for stocks that began more than a decade ago.

After a string of sharp losses, all major U.S. indices have now fallen more than 20% from their recent peaks.

Here are some common questions asked about bear markets and corrections and what they mean for average investors:

___

HOW IS A BEAR MARKET DIFFERENT FROM A MARKET CORRECTION?

A correction is Wall Street's term for an index like the S&P 500, the Dow Jones Industrial Average, or even an individual stock, that's fallen 10% or more from a recent high. A bear market occurs when the index or stock falls 20% or more from the peak for a sustained period of time.

Corrections are common during bull markets, and are considered normal and even healthy. They allow markets to remove speculative froth after a big run-up and give investors a chance to buy stocks at lower prices.

The major U.S. stock indexes entered a correction this month amid mounting fears about the impact that the coronavirus outbreak could have on the global economy and company earnings growth. A oil market price war this week that led analysts to lower their profit forecasts for energy companies fueled more selling on Wall Street.

On Thursday, the S&P 500 plummeted 9.5% to 2,480.64. It has plummeted 26.7% from its all-time high of 3,386.15 on Feb. 19.

The Dow had its worst day since the market crash of 1987, sinking 10% to 21,200.62. It is now 28.3% below its record close of 29,551.42 on Feb. 12.

The Nasdaq dropped 9.4% to 7,201.80, or 26.6% below its peak of 9,817.18 on Feb. 19.

___

WHAT'S BOTHERING INVESTORS?

The outbreak of the coronavirus that originated in China has quickly grown into a pandemic that is threatening major sectors of the global economy, stoking fear that the U.S. and other economies could be tipped into a recession.

Many companies, including airlines, cruise operators and big consumer technology manufacturers, have warned their earnings will take a hit this year due to the economic fallout from the outbreak.

Investors remain uncertain over whether action taken by the Federal Reserve and the Trump administration to shield the economy will be effective or arrive quickly enough to prevent widespread economic pain.

More recently, a sharp drop in crude oil prices has further dimmed the overall outlook for corporate profits this year and next. Company profits tend to be the biggest driver of stock market gains.

___

HOW OFTEN DO MARKET CORRECTIONS BECOME BEAR MARKETS?

In the S&P 500, there have been 23 corrections since 1945 and 12 bear markets, not including the current near-bear market, said Sam Stovall, chief investment strategist for CFRA. That works out to corrections becoming bear markets a little less than 35% of the time.

The current sell-off marks the fastest drop of 20% by the S&P 500 index on record, Stovall said.

___

WHEN WAS THE LAST TIME WE HAD A BEAR MARKET?

The last bear market for the S&P 500 ran from Oct. 9, 2007 through March 9, 2009. The index fell 56.8%. in that 17-month period as the U.S. housing downturn and mortgage crisis erupted, triggering a credit crunch.

___

HOW LONG DO BEAR MARKETS LAST AND HOW DEEP DO THEY GO?

On average, bear markets have lasted 14 months in the period since World War II, while market corrections have lasted an average of five months. The S&P 500 index has fallen an average of 33% during bear markets in that time. The biggest decline since 1945 occurred in the 2007-2009 bear market.

History shows that the faster an index enters into a bear market, the shorter they tend to be. Historically, stocks take 270 days to fall into a bear market. When the S&P 500 has fallen 20% at a faster clip, the index has averaged a loss of 26%, Stovall said.

___

WHAT ARE THE SIGNS THAT A CORRECTION OR A BEAR MARKET HAS ENDED?

Generally, investors look for a 20% gain from a low point as well as sustained gains over at least a six-month period.

On average, bull markets last 4.5 years. In terms of the S&P 500, the current bull market has been going on for almost 11 years.

The shortest bear market for the S&P 500 was in 1990. It lasted almost three months, sliding 20% in that period. The longest was a 61-month bear market that ended in March 1942 and cut the index by 60%.