A census blockis the smallest geographic unit used by the United States Census Bureaufor tabulation of 100-percent data (data collected from all houses, rather than a sample of houses). Several blocks make up block groups, which again make up census tracts. There are on average about 39 blocks per block group, but there are variations. Blocks are typically bounded by streets, roads or creeks. In cities, a census block may correspond to a city block, but in rural areas where, roads are fewer, blocks may be limited by other features. The population of a census block varies greatly.
Copyright statement: U.S. Census (http://www.census.gov/)