A comprehensive count of the population.
Censuses are resources which detail population characteristics with the greatest level of accuracy achievable in real life.
They often include measures of society that can be difficult to measure otherwise, such as: employment status, school enrollment, personal income, need of public assistance (due to being in poverty or disabled), family structure, migration, citizenship status, educational attainment, overcrowding, health insurance enrollment, and so on.
Note: If you are specifying the U.S. Census Bureau, make sure to include the USA tag as well. The same would go for the UK, Canada, Australia, etc. if your questions are with respect to those countries.