While economic theory in general may be defined as the theory of how an economic condition or an economic development is determined within an institutional framework, the welfare theory deals with how to judge whether one condition can be said to be better in some way than another and whether it is possible, by altering the institutional framework, to achieve a better condition than the present one.
Kenneth Arrow and John Hicks (1972) From Nobel Lectures, Economics 1969-1980, Editor Assar Lindbeck, World Scientific Publishing Co., Singapore, 1992 (online).