Source: U.S. Bureau of Economic Analysis  

Release: Gross Domestic Product  

Units:  Percent Change from Preceding Period, Seasonally Adjusted Annual Rate

Frequency:  Quarterly

BEA Account Code: PB000009

The arithmetic average of gross domestic product and of gross domestic income, deflated by the implicit price deflator for GDP.
A Guide to the National Income and Product Accounts of the United States (NIPA) - (http://www.bea.gov/national/pdf/nipaguid.pdf)