The United States Census is when the population of the United States of America is counted every 10 years by the Census Bureau. The count is based on the number of people living in residential structures and includes citizens, non-citizen legal residents, non-citizen long term visitors AND illegal immigrants. 

Although the Census Bureau tries to ensure those without conventional housing are counted, the numbers are not considered particularly accurate. American citizens who live overseas are excluded from the national census but may still continue to vote. American citizens who live overseas and are federal employees (military & civilian) ARE, however, counted in the census. The next census will be in 2020.