The National Bureau of Economic Research said Monday the United States entered into a recession after the economy peaked in December 2007. That month marked the end of an expansion that began in November 2001.

A traditional definition of a recession calls for two consecutive quarters of contraction in real gross domestic product. However, the bureau said its definition of a recession is a period of "diminishing" activity.

"We identify a month when the economy reached a peak of activity and a later month when the economy reached a trough," according to the bureau. "The time in between is a recession, a period when economic activity is contracting."