A revolution (from Latin revolutio which means "a turn around") is a significant change and often (previously) illegal alteration in government and in related associations and structures that usually occurs in a short period of time. By analogy, "revolution" is used in terms for other major transformation, such as the Industrial Revolution, although such transformations need not be illegal or very quick. Regarding the difference with a coup d'état, see the article on this topic.
Variously defined revolutions have happened throughout human history and continue today. They vary in terms of numbers of their participants (revolutionaries), means employed by them, duration, motivating ideology and many other aspects. They may result in a socio-political change in the socio-political institutions, or a major change in a culture or economy.
Scholarly debates about what is and what is not a revolution center around several issues. Early study of revolutions primarily analyzed events in European history from psychological perspective, soon however new theories were offered using explanations for more global events and using works from other social sciences such as sociology and political sciences.
If a group, typically mercenaries, come from outside a country to start a revolution and poses as falsely rebels from within the country, it is a fake revolution.