1. A terrible disaster or accident; especially, one that leads to a great loss of life.
2. A total or absolute failure, often in humiliating or embarrassing circumstances.
3. In geology, a violent seismic change; a sudden and violent change in the Earth's crust caused by an earthquake, flood, or any other natural process; a cataclysm.
4. An insurance event causing huge insurance claims, or an event causing losses of insured property above a specific monetary limit and affecting a substantial number of policyholders and insurers.
5. Etymology: from the 1530's, "reversal of what is expected"; especially, a fatal turning point in a drama, from Latin catastropha, from Greek katastrophe, "an overturning; a sudden end"; from katastrephein, "to overturn, to turn down, to trample on; to come to an end"; from kata "down" + strephein "to turn" and directly related to Greek strophe, originally "a turning" with reference to the section of an ode sung by a chorus while turning in one direction. The extension to "sudden disaster" is first recorded in 1748.