Census

n.
  1. (Bot. Antiq.) A numbering of the people, and valuation of their estate, for the purpose of imposing taxes, etc.; -- usually made once in five years.
  2. An official registration of the number of the people, the value of their estates, and other general statistics of a country.

A general census of the United States was first taken in 1790, and one has been taken at the end of every ten years since.