Wisdom has been a topic of religion and philosophy since the dawning of human civilization. But only during the last two or three decades wisdom has become a topic of empirical research in developmental psychology, adult and old age education, as well as in management and leadership studies. The aim of this paper is to elaborate a new definition of "wisdom," in order to provide a more adequate foundation for empirical wisdom research. To reach this goal, two empirical wisdom theories (Baltes and Ardelt) and two philosophical wisdom approaches (Ryan and Whitehead) are presented, discussed, compared, and synthesized. The results show that despite the fact that Baltes' definition of "wisdom" is somewhat wider than Ardelt's, their approaches have many aspects in common. Additionally, also Whitehead's ideas on wisdom are quite similar to Baltes' core criteria concerning wisdom-related knowledge, but Whitehead mentions two additional aspects which go beyond Baltes' approach. Further, according to Ryan, wise persons must have very few unjustified beliefs; this necessary condition for a wise person is neither mentioned by Baltes, nor by Ardelt or Whitehead. Based on the ideas of these four approaches, a new definition of "wisdom" is presented, in which wisdom is relativised to time indices and moral systems. This definition consists of a cognitive, an ethical, and a reflective component. Further it is shown that Fischer's and Dawson's "Lectical Assessment System" is a major candidate for measuring wisdom on the basis of our new definition.