What term is used for a person who actively supports or advocates for radical changes in society?

The term used for a person who actively supports or advocates for radical changes in society is "Revolutionary." This term specifically refers to individuals who seek significant, often systemic, changes to the existing social or political structures. Revolutionaries are typically characterized by their desire for fundamental transformations, which can involve overthrowing the current regime or system.

In contrast, while "Activist" might also suggest someone who advocates for change, it does not imply the level of radical transformation that "Revolutionary" does. Activists can work towards reforms within the existing system rather than pursuing a complete overhaul. Similarly, "Reformist" refers to those who advocate for gradual changes and improvements within the existing framework rather than advocating for radical changes, and "Conservative" typically describes a person who prefers to maintain traditional values and resist significant changes. Thus, "Revolutionary" best captures the essence of advocating for radical societal changes.