“The purpose of this change is simply to provide more information than we have in the past, even though it is qualitative information, as the unemployment rate declines below 6.5 percent,” Ms. Yellen said.

But the Fed also released a separate set of economic forecasts showing that officials had raised their expectations for the level of their benchmark rate at the end of 2015 to 1 percent from 0.75 percent.

And, when asked to define “a considerable time,” Ms. Yellen responded, “This is the kind of term that’s hard to define, but it probably means something on the order of around six months or that kind of thing.” She added that the exact timing of a rate increase would depend on economic conditions. Investors, however, focused on the words “six months,” which suggested a first rate increase in the spring of 2015 — six months after the expected end of bond purchases, and considerably earlier than the timing they previously expected.

The stock market fell on her words. The S.&P. 500 was down 0.61 percent on the day. The yield on the benchmark 10-year Treasury note rose to 2.77 percent.

“It is hard to know if Yellen wanted the market, which has taken it hard on the chin following that particular remark, to start thinking of a mid-2015 hike, but that is what was implied by her statement,” Millan L. Mulraine, deputy head of research at TD Securities, wrote in a note to clients on Wednesday afternoon.

The strategy shift has prompted internal debate.

Narayana Kocherlakota, president of the Federal Reserve Bank of Minneapolis, and one of the most ardent proponents of the view that the Fed is not doing enough to stimulate the economy, voted against the new guidance. He has argued that the Fed should adopt a new unemployment threshold of 5.5 percent. Mr. Kocherlakota said that the Fed’s move called into question its commitment to reviving inflation and created uncertainty “that hinders economic activity.”