However, the government's definition of poverty is not tied to an absolute value of how much an individual or family can afford, but is tied to a relative level based on how much the average individual makes. Most Americans (58.5%) will spend at least one year below the poverty line at some point between ages 25 and 75. There remains some controversy over whether the official poverty threshold over- or understates poverty.

The most common measure of poverty in the United States is the "poverty threshold" set by the U.S. government. This measure recognizes poverty as a lack of those goods and services commonly taken for granted by members of mainstream society. The official threshold is adjusted for inflation using the consumer price index.

Relative poverty describes how income relates to the median income, and does not imply that the person is lacking anything. In general the United States has some of the highest relative poverty rates among industrialized countries, reflecting both the high median income and high degree of inequality. In terms of pre-transfer absolute poverty rates, in 2000 the United States ranked tenth among sixteen developed countries, though 2000 was a 'trough' year and subsequently absolute poverty rates have increased.. The US does worse in post-transfer absolute poverty rates. According to a 2008 report released by the Carsey Institute at the University of New Hampshire, on average, rates of poverty are persistently higher in rural and inner city parts of the country as compared to suburban areas.

Measures of poverty can be either absolute or relative.

There are two basic versions of the federal poverty measure: the poverty thresholds (which are the primary version) and the poverty guidelines. The Census Bureau issues the poverty thresholds, which are generally used for statistical purposes—for example, to estimate the number of people in poverty nationwide each year and classify them by type of residence, race, and other social, economic, and demographic characteristics. The Department of Health and Human Services issues the poverty guidelines for administrative purposes—for instance, to determine whether a person or family is eligible for assistance through various federal programs.

Since the 1960s, the United States Government has defined poverty in absolute terms. When the Johnson administration declared "war on poverty" in 1964, it chose an absolute measure. The "absolute poverty line" is the threshold below which families or individuals are considered to be lacking the resources to meet the basic needs for healthy living; having insufficient income to provide the food, shelter and clothing needed to preserve health.

The "Orshansky Poverty Thresholds" form the basis for the current measure of poverty in the U.S. Mollie Orshansky was an economist working for the Social Security Administration (SSA). Her work appeared at an opportune moment. Orshansky's article was published later in the same year that Johnson declared war on poverty. Since her measure was absolute (i.e., did not depend on other events), it made it possible to objectively answer whether the U.S. government was "winning" this war. The newly formed United States Office of Economic Opportunity adopted the lower of the Orshansky poverty thresholds for statistical, planning, and budgetary purposes in May 1965.

The Bureau of the Budget (now the Office of Management and Budget) adopted Orshansky's definition for statistical use in all Executive departments. The measure gave a range of income cutoffs, or thresholds, adjusted for factors such as family size, sex of the family head, number of children under 18 years old, and farm or non-farm residence. The economy food plan (the least costly of four nutritionally adequate food plans designed by the Department of Agriculture) was at the core of this definition of poverty.

The Department of Agriculture found that families of three or more persons spent about one third of their after-tax income on food. For these families, poverty thresholds were set at three times the cost of the economy food plan. Different procedures were used for calculating poverty thresholds for two-person households and persons living alone. Annual updates of the SSA poverty thresholds were based on price changes in the economy food plan.

Two changes were made to the poverty definition in 1969. Thresholds for non-farm families were tied to annual changes in the Consumer Price Index (CPI) rather than changes in the cost of the economy food plan. Farm thresholds were raised from 70 to 85% of the non-farm levels.