What is a census tract?

Per the United States Census Bureau, a census tract is a small, relatively permanent statistical subdivision of a county. They are uniquely numbered and average about 4,000 inhabitants, with a minimum size of 1,000 inhabitants and a maximum size of 8,000 inhabitants. 
 
You can search for a specific census tract on the United States Census Bureau's lookup site: https://tigerweb.geo.census.gov/tigerweb/