1. （尤指战争引起的）大屠杀，大毁灭，浩劫
A holocaust is an event in which there is a lot of destruction and many people are killed, especially one caused by war.

2. （第二次世界大战中纳粹对犹太人进行的）大屠杀
The Holocaust is used to refer to the killing by the Nazis of millions of Jews during the Second World War.

1. an act of mass destruction and loss of life (especially in war or by fire)

e.g. a nuclear holocaust