IPA: //ˈkætəˌklɪzəm//
KK: /ˈkætəˌklɪzəm/
A sudden and violent event that causes significant destruction or change, often affecting a large area or population.
The earthquake was a cataclysm that changed the landscape forever.
Cataclysm → It is formed from "cata-" (from Greek "kataklyzein", meaning to wash down or overflow) and "klysmos" (meaning a washing or flood). The word describes a significant and often violent event that causes a great upheaval or disaster, akin to a flood washing away the old.
Think of a great 'flood' or 'overflow' ('cata-') that causes a significant change or disaster, which is what a cataclysm represents.
No commonly confused words.