The analytical concept describing the sudden, fundamental transformation of a political system — typically through popular mobilization, the overthrow of existing authority, and the seizure of state power — and the framework scholars use to explain why revolutions happen, how they develop, and what they produce.
Every major political transformation of the last three centuries has been called a revolution, or has had to explain why it wasn't one. The concept does real work — it distinguishes fundamental change from reform, legitimate succession from violent seizure — but it also creates its own mythology, one that shapes what revolutions do once they begin.