That branch of economics concerned firstly with defining economic efficiency; secondly with evaluating the economic efficiency of particular systems of resource allocation; and finally with analysing the conditions under which economic policies may be said to have improved social welfare. The first two sets of problems are approached by defining the conditions which are necessary for a maximum of economic efficiency to be achieved, and then by examining the degree to which actual systems, e.g. the price system, fulfil these conditions. The third set of problems is approached by defining tests or criteria which may be applied to proposed changes to decide if they represent improvements in welfare or not.
Reference: The Penguin Dictionary of Economics, 3rd edt.