The population of a given area is defined as the number of people usually living in that area, measured on 1 January in a given year. The source can be the most recent population census (a census is when the population is counted). ... In some countries population registers are used instead.

How is the population counted?

The U.S. Census Bureau is the leading source of statistical information about the nation's people. Our population statistics come from decennial censuses, which count the entire U.S. population every ten years, along with several other surveys.

What does a population do?

A population is a distinct group of individuals, whether that group comprises a nation or a group of people with a common characteristic. In statistics, a population is the pool of individuals from which a statistical sample is drawn for a study.