the total of the squared income/expenditure shortfall (expressed in proportion to the poverty threshold) of families/ individuals with income/expenditure below the poverty threshold, divided by the total number of families/ individuals  
Notes:
This is equal to the Foster-Greer-Thorbecke (FGT) family of measures with alpha = 2. It is a poverty measure, which is sensitive to the income/ expenditure distribution among the poor - the worse this distribution is, the more severe poverty is.