(ECONOMICS) an official definition of poverty, in which one third of one's income is spent on food. "Food," here, is defined as the most cost-effective way of meeting basic nutritional needs.
The definition has one advantage, which is that researchers can get comparable information about poverty for any country in the world. The disadvantage is that it's arbitrary (why one third? why food? why not shelter, health care, and heating?); the other is that the cost of living varies dramatically in different neighborhoods in different cities of different US states, yet the poverty level is the same (expressed in dollar amounts) everywhere in a given country.
A better measure is the self-sufficiency standard.
Living under the official poverty level can be a lot worse in affluent communities like San Francisco, where the cost of basic necessities is very high. On the other hand, it's also a lot worse in areas such as rural Mississippi, where public amenities (such as libraries equipped with computers for public use) are rare.