Statistical areas bounded by visible features such as roads, streams, and railroad tracks, and by nonvisible boundaries such as property lines, city, township, school district, county limits and short line-of-sight extensions of roads.
The building blocks for all geographic boundaries the Census Bureau tabulates data for, such as tracts, places, and American Indian Reservations.
Generally small in area. In a city, a census block looks like a city block bounded on all sides by streets. Census blocks in suburban and rural areas may be large, irregular, and bounded by a variety of features, such as roads, streams, and transmission lines. In remote areas, census blocks may encompass hundreds of square miles.
A wall-to-wall coverage across the entire territory of the United States, Puerto Rico, and the Island Areas.
Numbered uniquely with a four-digit census block number ranging from 0000 to 9999 nesting within each census tract, which nest within state and county. The first digit of the census block number identifies the block group. Block numbers beginning with a zero (in Block Group 0) are associated with water-only areas.
Delineated by the U.S. Census Bureau once every ten years. An automated computer process looks for all visible and nonvisible features in our geographic database (MAF/TIGER) that should be a block boundary and creates a block each time those features create a polygon.
The smallest level of geography you can get basic demographic data for, such as total population by age, sex, and race.

Census blocks are not:

Delineated based on population. In fact, many census blocks do not have any population.
Permanent throughout the decade. They may be split when a change in another geographic boundary occurs, such as an incorporated place annexation. If a block is split in between decades, a suffix will be added to the block number. For example, block 1000 would become block 1000A and 1000B.
A boundary that can be used with American Community Survey (ACS) data. ACS data only go down to the block group level.

If you’d like to learn more, visit these links to block-related products created by the U.S. Census Bureau:

P.L. 94-171 County Block Maps (2010 Census)

These large-scale maps show the boundaries and numbers for all census blocks within a county.

TIGER/Line® Shapefiles and TIGER/Line® Files

All legal boundaries and names are as of January 1, 2010. Released beginning November 30, 2010. Last update March 26, 2012.

2010 Census Block Assignment Files (BAF)

BAFs are among the geographic products provided to states and other data users containing the small area census data necessary for legislative redistricting.

Block Relationship Files

2010 Census Block Relationship files are provided as a tool to help data users compare the universe of Census 2000 blocks to the universe of 2010 Census blocks.

Geography Page

Geography is central to the work of the Census Bureau, providing framework for survey design, sample selection, data collection, tabulation, and dissemination.