a violent geological change of the earth's surface
a momentous event marked by violent upheaval and destruction
a flood or deluge
cataclysmal /-ʹklizməl/ adj
cataclysmic /-ʹklizmik/ adj
cataclysmically /-ʹklizmikli/ adv
[French cataclysme via Latinfrom Greek kataklysmos, from kataklyzein to flood, from kata- cata- + klyzein to wash]