The dictionary lists "flat" and 'smooth" as synonyms, but anyone who followed the stock market last year could clearly see the difference.

Market historians will record 2011 as a flat year, with the Standard & Poor's 500 index falling a microscopic 0.003 percent, but the charts show a ride that was anything but smooth. Stocks started the year strong, then fell nearly 20 percent between late April and early October before rallying late in the year.

That pattern isn't unusual: On average since 1900, the market has had at least one correction per year that knocked at least 10 percent off of stock prices. What made 2011 different — and sometimes excruciating — was the frequency of big, one-day gains and losses.

Last year, the Dow Jones industrial average rose or fell more than 2 percent on 32 separate days. That included an unprecedented streak in August when the Dow gained or lost at least 400 points on four consecutive days.

Such volatility is common during severe bear markets, like the one of 2008 and early 2009, but usually doesn't occur when the market is moving sideways.

For much of the year, investors were reacting to headlines from Europe, where leaders constantly seemed to be letting the financial system collapse one day and finding a way to save it the next. The ups and downs created a sense of helplessness for U.S.-based investors, who saw their 401(k) balances swing wildly in reaction to distant events they only dimly understood.

If it's any comfort, last year's volatility wasn't kind to hedge fund managers or actively managed mutual funds, either. The majority of them failed to keep up with their market benchmarks.

"Trying to develop, on the fly, a lot of expertise in German regional politics or Italian bond auctions is going to be tough," says Norman Conley, chief investment officer at JAG Advisors in Ladue.

For individual investors, the best advice is to try to understand the events in Europe, but don't react to them. Trying to time a volatile market is generally a loser's game: If you get spooked and sell after a big downdraft, you probably won't screw up the courage to buy stocks again for a while, and you might miss out on big gains.

It's better to turn off the TV when the announcers start talking about the latest panic. If your investment strategy was sound to start with, you'll probably be OK when the shouting is over. Those four crazy days in August, for example, went loss, gain, loss, gain, and the net result left the Dow just 1 point below where it began.

"That level of volatility makes investors very nervous," says Alan Skrainka, chief investment officer at Cornerstone Wealth Management in Des Peres, "but you need to focus on the things you can control."

That means sticking with high-quality investments and making sure you're properly diversified. You can even make volatility work in your favor by rebalancing your portfolio periodically.

If you started with a portfolio that was 60 percent in stocks and 40 percent in bonds, a nosedive in the market might have temporarily changed that proportion to 55 to 45 percent. You'd rebalance by selling some bonds and buying more stock — at depressed prices.

It doesn't matter if your crystal ball is hazy; no investor knows what the future holds. As they look at the year ahead, investment professionals' best guess is that we're in for more of the same.

"I think we'll continue to see day-to-day and week-to week volatility," says Stuart Freeman, chief equity strategist at Wells Fargo Advisors in St. Louis.

The bear market of 2008-09 was so traumatic that investors are still looking over their shoulders and worrying that something similar may happen again. What many people forget is that we've just lived through a strong bull market, too, with stock prices doubling between March 2009 and April 2011.

A cooling-off period is normal after that kind of a gain, Freeman says. "It's not too unusual, once you're past the first move up after a recession, to get 16 to 18 months of flatness, which a lot of investors don't expect."

Freeman thinks the market could keep going sideways for the first six months or so of the new year.

With European finances still tenuous, though, and the U.S. gearing up for a presidential election, investors navigating this flat market are likely to keep encountering plenty of bumps.