Being well-fed in the United Kingdom is probably not the same as being well-fed in say... Laos.
That is certainly true - the food is much better, tastier, and healthier in Laos.
Of course a dollar amount to designate poverty is ridiculous.. jmfcst's suggestion of enough to eat, housing, and clothing is not unreasonable, though one must add 1) security - the sense that these things are not about to be lost, knowing from whence one's next meal will come, etc., and 2) a sense of opportunity, progress, improvement.
By these measures I would say about half of the population of mose countries lives in 'poverty', including the US.
^ ^ ^
Except for the last sentence. I've seen the line "90% of the third world and 10% of the first world" used, and it's not that unreasonable. Probably cut 90% to 80% or 75%, and allow an intermediary category for places like Chile, Mexico, Taiwan...
