The error in census data that results from the failure to count some persons or housing units in the decennial census. Historically, certain groups, such as members of racial or ethnic minorities, have been disproportionately undercounted by the federal census.

The estimated number of people who are not counted by the Census.

The failure to count all the persons or events which should be counted in a census or vital statistics registration system. Census undercount (underenumeration) may occur due to persons not wishing to be included (even though it is a legal requirement in virtually all countries) or through the difficulty of tracking down people in some areas. Estimates of census undercount are normally made through a post enumeration survey. Birth, death and marriage undercounts occur due to inadequate registration systems.