What term describes a sudden overthrow of the government by a small group?

The term that describes a sudden overthrow of the government by a small group is "Coup d'état." This term specifically refers to an illegal and often violent seizure of power, typically executed by a group within the existing government or military. A coup is generally characterized by its swift and clandestine nature, aiming to replace the current leadership without the involvement of a broader segment of the population.

While "putsch" can also refer to a sudden attempt to overthrow a government, it is a term that is more commonly used in specific historical contexts, particularly in relation to Germany and Austria. "Rebellion" and "insurrection" denote broader uprisings involving larger groups seeking to challenge authority or government, often with the aim of significant systemic change rather than a quick power grab by a small faction. Therefore, coup d'état is the most precise term for the scenario described, emphasizing the nature of a swift and targeted action against an existing regime.