Census blocks are the smallest geographic area for which the Census Bureau collects and tabulates decennial census data. These units are formed by streets, streams and other bodies of water, other visible physical and cultural features and the legal boundaries shown on Census Bureau maps. In 1990, the Census Bureau tabulated data for 7,020,924 census blocks.
the smallest area for which the Census Bureau collects and tabulates decennial census data. A geographic area normally bounded on all sides by visible features such as streets, roads, streams and railroad tracks, and occasionally by non-visible boundaries such as city or county limits, property lines, and short, imaginary extensions of streets or roads.
A subdivision of a census tract, a block is the smallest geographic unit for which the Census Bureau tabulates 100-percent data. Many blocks correspond to individual city blocks bounded by streets, but blocks -- especially in rural areas - may include many square miles and may have some boundaries that are not streets. The Census Bureau established blocks covering the entire nation for the first time in 1990. Previous censuses back to 1940 had blocks established only for part of the nation. Over 8 million blocks are identified for Census 2000. Census Geography: A Quick Primer (PDF 123kb)