Census Block
The smallest geographic unit used by the Census Bureau, typically bounded by streets, streams, or other visible features, for which only total-population counts are published.
Census blocks are the building blocks (literally) of all higher Census geographies. Each block is a small area bounded by visible features like streets, railroad tracks, and streams, or by invisible boundaries like city limits and property lines. There are approximately 11 million Census blocks in the United States, identified by a 4-digit code within their parent block group.
Due to their small size, Census blocks are the unit used to construct ZCTAs: the Census Bureau assigns each block to the ZIP code most commonly found in its addresses and then aggregates blocks into ZCTA polygons.
Only limited data (total population and housing unit count) is published at the block level from the decennial Census. For detailed demographics, users must use block groups or tracts.