The U.S. Census Bureau is one of the federal agencies that calculate poverty thresholds and publishes data on the poverty status of the population. You can find an overview of their methodology and terms at
There are actually 48 possible thresholds size of the family and ages of the members. For two people, the figures can change depending on whether there is one adult and one related child under age 18 and whether the adult is aged 65 years and older or less than 65. Similarly, if both people are adults, then the figures can change based on whether the adults are under 65 or 65 and over. You can find the poverty thresholds by year online at www.census.gov/hhes/www/poverty/threshld.html.
Poverty guidelines are also published by the U.S. Department of Health and Human Services at aspe.hhs.gov/poverty/index.shtml. These guidelines are used to determine eligibility for many federal programs.