How does Ehrenreich define the concept of living wage and explain why is it difficult to accomplish at the Federal level in Nickel and Dimed?
Ehrenreich refers to a "living wage" very early on in her book (page 3) but only loosely defines it as a wage that allows you to survive economically, pay for basic needs such as food, shelter, medical care, etc. without any form of government assistance. The entire book is an experiment with living wages, so she implicitly refers to this almost throughout her story.
A living wage is very difficult to accomplish at the federal level because you cannot legislate it. A living wage in Hope, Arkansas can be much less than a living wage in San Francisco, where the cost of housing and gas are much higher. So a "minimum wage" often has little to do with a "living wage".
In an interview with PBS, Ehrenreich cites a study by the Economic Policy Institute in Washington DC that claims a living wage to be $14 an hour, but she also points out this is only a national average and, again, could not be applied as law either practically or politically.