What is it that unites and defines a variety of people that are called "dissidents"—in the media, political debates or academic literature? This chapter provides a compendium of five definitions, gathered through a board literature review and interviews with former human rights activists and political oppositionists. It then traces the evolution of the meaning of the dissident from its religious root to its first secular and political applications. Further, it clarifies the conceptualization of three closely connected terms: dissent, dissidence, and dissidentism.