(1) A violent and massive change of the earth's surface. (2) A momentous event that results in great upheaval and often destruction.
Example:
World War I was a great cataclysm in modern history, marking the end of the European social and political order.
A cataclysm causes great and lasting changes. An earthquake or other natural disaster that changes the landscape is one kind of cataclysm. We might also speak of the cataclysmic changes brought by a political revolution. Even a new discovery or invention can be seen as cataclysmic if it brings great changes in how people think or work.