The informal definition of a recession is two consecutive quarters of negative economic growth. The National Bureau of Economic Research - a private organization of academic economists - is, however, the official arbiter of when recessions begin.
The NBER designates the beginning of a recession months after it has started and designates its ending months (or sometimes years) after it has ended. According to NBER, a recession is "a significant decline in economic activity spread across the economy, lasting more than a few months, normally visible in real GDP, real income, employment, industrial production, and wholesale-retail sales. A recession begins just after the economy reaches a peak of activity and ends as the economy reaches its trough. Between trough and peak, the economy is in an expansion," the normal state of the economy.
The NBER has reviewed historical and current economic data to designate 33 American economic recessions since 1854.
The most recent recession lasted from December of 2007 to June of 2009, according to NBER. The Business Cycle Dating Committee of the NBER determined in late September of 2010 that a trough in business activity occurred in the U.S. economy in June of 2009. According to the NBER, the trough marked the end of the recession that began in December 2007 and the beginning of an expansion. The recession lasted 18 months, which makes it the longest of any recession since World War II. Previously the longest postwar recessions were those of 1973-75 and 1981-82, both of which lasted 16 months.
The previous recession lasted from March 2001 through November 2001, but it wasn’t until November 2001 that the NBER pinpointed March 2001 as the beginning of the recession; and it wasn’t until July 2003 that it determined precisely when the recession had ended.