Here's how the USDA explains the term:

The HFFI working group defines a food desert as a low-income census tract where a substantial number or share of residents has low access to a supermarket or large grocery store:

To qualify as a “low-income community,” a census tract must have either: 1) a poverty rate of 20 percent or higher, OR 2) a median family income at or below 80 percent of the census tract's median family income;
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).

As part of First Lady Michelle Obama's "Let's Move" initiative, the Healthy Food Financing Initiative - a joint effort of the Treasury Department, Health and Human Services, USDA along with along with staff from the Economic Research Service - will "expand the availability of nutritious food to food deserts—low-income communities without ready access to healthy and affordable food—by developing and equipping grocery stores, small retailers, corner stores, and farmers markets with fresh and healthy food," according to the USDA's website.

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