The Business Cycle Dating Committee of the National Bureau of Economic Research (NBER) formally announces that the US economy has been in recession since December 2007, making the present downturn already longer than the average for all recessions since World War II.
The NBER defines a "recession as a significant decline in economic activity spread across the economy, lasting more than a few months, normally visible in production, employment, real income, and other indicators. A recession begins when the economy reaches a peak of activity and ends when the economy reaches its trough. Between trough and peak, the economy is in an expansion."
While the Committee generally uses the two continuous quarters of declining real economic activity (GDP) as a thumb rule, it does not go by this yardstick alone. Interestingly, the Committee defines recession as "a period of diminishing activity rather than diminished activity".