The chapter addresses the question of the definition of a Jewish collectivity as it was formed in Hellenistic and Roman times by Jews. Having a single Hebrew term to designate themselves, Bney Israel (“the sons of Israel”), Jews had to do without concepts such as ethnos, genos, laos, dēmos, populus, natio, polis, and civitas when referring to themselves as a collective group. The chapter examines the notions that Jews used in order to refer to themselves as an entity, and shows that the definition of Judaism by Jews was modeled in view of different concepts of other entities that were predominant in the Greco-Roman world and was influenced by the tension between political, geo-ethnic, historical, juridical and civic definitions. Each type of collective definition served a different realpolitik and was conditioned by different political circumstances, which determined the way in which Jews demarcated themselves as a group. The chapter aims to reveal the evolution that the definition of Judaism underwent in a period of great changes and focuses in particular on the transition between the Hellenistic and Roman periods.