please take a look at this US Census definition:

'First generation' refers to those who are foreign born; 'second generation' refers to those with at least one foreign-born parent; 'third-and-higher generation' includes those with two U.S. native parents. Note: Numbers in thousands. Universe is the civilian noninstitutionalized population of the United States, plus armed forces living off post or with their families on post. Source: U.S. Census Bureau, Current Population Survey, Annual Social and Economic Supplement, 2007.

available at:

http://www.census.gov/population/foreign/files/cps2007/tab4-2007.pdf