From Census.gov > Topics > Population > Ancestry > About: Ancestry refers to a person’s ethnic origin or descent, “roots,” or heritage, or the place of birth of the person or the person’s parents or ancestors before their arrival in the United States. Some ethnic identities, such as “German” or “Jamaican,” can be traced to geographic areas outside the United States, while other ethnicities, such as “Pennsylvania Dutch” or “Cajun,” evolved in the United States.

From the American Community Survey, Why We Ask Questions About... Ancestry: We ask a question about a person’s ancestry or ethnic origin to create statistics about ancestry groups in America.

Local, state, tribal, and federal agencies use data about ancestry to plan and evaluate government programs and policies to ensure that they fairly and equitably serve the needs of all groups. These statistics also help enforce laws, regulations, and policies against discrimination in society.

Source: 2019 American Community Survey (ACS), 1-Year Estimates.

Note: The table above is modified to show reported ancestries that total 1 million or more. The top five are highlighted (not including Other groups and Unclassified or not reported). The top five are German, Irish, English, American and Italian — each with more than 10 million. Click on the image to see the full, original table.

Source: 2019 American Community Survey (ACS), 1-Year Estimates.

From Topics > Population: 

Source: 2010-2014 and 2009-2013 American Community Survey (ACS), 5-Year Estimates.

Here are two graphics from the previous decade about two of the largest ancestry groups: German and Irish.

From Census.gov > History: 

From the Newsroom: