There is no simple definition of poverty; the two most common definitions used are as follows:
Absolute poverty is based on the idea that there is a subsistence level of income necessary to sustain life. It is usually measured by calculating the price of the absolute necessities of life. Anyone whose income is insufficient to provide the necessities of life is said to be in poverty. The necessities are assumed to include nutrition, shelter and health. Nutrition, for example is measured by working out diet sheets that cover minimum protein and calorie intake.
Relative poverty defines poverty in terms of the standards of living in a particular society at a given time. It does not start from an absolute minimum subsistence level. What counts as poverty in relative terms changes overtime. Lack of a refrigerator might be seen as an indicator of poverty in Western societies in the 1990s but would certainly not have been in the 1940s. Relative poverty is measured in terms of the level of affluence of the society at a given point in time.