  Recently Qiu et al. (2017) have introduced residual extropy as measure of
uncertainty in residual lifetime distributions analogues to residual entropy
(1996). Also, they obtained some properties and applications of that. In this
paper, we study the extropy to measure the uncertainty in a past lifetime
distribution. This measure of uncertainty is called past extropy. Also it is
showed a characterization result about the past extropy of largest order
statistics.
