Just guessing here, but it would seem reasonable to define extreme poverty as deprivation that substantially impedes one's ability to maintain a decent life exp

Just guessing here, but it would seem reasonable to define extreme poverty as deprivation that substantially impedes one's ability to maintain a decent life expectancy. I might also include very poor access to (say) basic education. 'Poverty' is a rather broader definition, including issues like housing affordability, decent food variety, access to secondary/higher education, perhaps life stability (for example, no access to permanent employment).