A census block is the smallest geographic unit used by the United States Census Bureau for tabulation of 100-percent data (data collected from all houses, rather than a sample of houses).
The number of blocks in the United States, including Puerto Rico and other island areas, for the 2020 Census was 8,180,866.
[2] Blocks are typically bounded by roads and highways, town/city/county/state boundaries, creeks and rivers, etc.
In cities, a census block may correspond to a city block, but in rural areas where there are fewer roads, blocks may be delimited by other features such as political boundaries, rivers and other natural features, as well as parks and similar facilities, etc.
Before that, back to the 1940 census, only selected areas were divided into blocks.