Usually determined by seeing what proportion of a family's income ends up spent on food and rent. If you are below this line, you are wasting your time at work because you're going to be in the exact same position when it ends as you were before you had it. Hand-to-mouth living.

According to the US Census Bureau, the 1999 poverty threshold (by yearly salary) looks something like this for households with no dependants:

Single person (under 65): $8667
Two person household (under 65): $11,156
Three persons: $13,032
Four persons: $17,184
Five persons: $20,723

The first poverty line was set in 1969 by LBJ. It took a minimal food budget and multiplied it by 3. Mollie Orshansky, "inventor" of the poverty measure, wrote a report in 1963 called "Children of the Poor," which had a crude set of guidelines.