Food deserts are vapid of fresh fruit, vegetables, and other healthy foods. They are also places that are considered "low-access communities" by the USDA. To qualify as a “low-access community,” at least 500 people and/or at least 33 percent of the census tract's population must reside more than one mile from a supermarket or large grocery store (for rural census tracts, the distance is more than 10 miles).
More info at the the Wikipedia article.
Even though I'm in a rural area, I'm about 5 1/2 miles from a WalMart Super Center.