Definitions are key elements of the scientific undertaken (journey) (Swedberg, Eur J Soc Theor 23(3):431–445, 2016). Definition consists of the tracing of a boundary of meaning, in order to distinguish what is inside and outside a specific research field. Definitions change along space and time. In the field of civil society organizations internationally many definitions have been developed world, recently there has been the tendency to gather the studies and research concerning the organized part of Civil Society under the label Third Sector. That seems to be more neutral and more easily recognized worldwide. The chapter is structured as follows. The first section introduces the topic of definition in social sciences. The second section deals with epistemological issues arising when dealing with the tough question of definitions. In the third section we will illustrate and comment on four main approaches to defining this research field. Finally, the fourth and last section presents concluding remarks.