The National Bureau of Economic Research (NBER), a nongovernmental organization, decides on the beginning and ending dates of recessions. A useful guide is a recession is defined to be at least two consecutive quarters of decline in real GDP, but the actual dating is subject to judgment by the committee.
[link to old NBER data]
Taxonomy of Actual Business Cycles
Business Cycles, Christina D. Romer, Concise Encyclopedia of Economics.
Recessions, Geoffrey H. Moore, Concise Encyclopedia of Economics.
What are business cycles and how do they affect the economy? (05/2002), Federal Reserve Bank of San Francisco.