NEW YORK — The stock market had its worst day of the year Thursday, just 24 hours after recording its best.

The Dow Jones industrial average plunged 334 points as falling energy stocks and worries about the global economy sent investors fleeing out of the market. The blue-chip index rose 275 points the day before.

For three years, U.S. investors have enjoyed a stock market that has, for the most part, quietly and steadily moved higher. The pleasure cruise appears to be over.

Market volatility is back and in a big way, market observers say. The stock market hasn’t seen day-to-day movements like this since August 2011, when Standard & Poor’s downgraded the United States’ credit rating. The S&P downgrade subsequently pushed the U.S. stock market into its last “correction,” a technical term for when stocks fall 10 percent or more from a recent peak.

“Investors are not conditioned for this type of market after three good years,” said Dean Junkans, chief investment officer for Wells Fargo Private Bank. “We’ve been long overdue for a correction.”

Words like “correction,” “fear” and “volatility” might scare the average investor just trying to save for retirement. But investors who might be worried should remain calm, said Jurrien Timmer, director of global macro at Fidelity Investments. The S&P 500 index is still up 4.3 percent this year. “Just stick to your long-term (retirement) plan,” Timmer said.

On Tuesday the Dow fell 272 points, only to jump by nearly the same amount Wednesday. Moves of 200-plus points had been rare until this week. More than half of this year’s 200-point moves happened in the last two weeks.