Holocaust, from Greek holo-kauston "all burnt", in its original sense refers to a completely-burnt sacrifice, see Holocaust (sacrifice).
By extention the word was used in English to refer to other acts of obliteration by burning. The specific sense of "massacre, destruction of a large number of persons" is attested from 1833, usually in a context in which victims are burnt to death or their bodies destroyed. It was in this sense that the term "nuclear holocaust" came to be used after World War II, to refer to the destruction of Hiroshima and Nagasaki and the anticipated result of a nuclear war and nuclear winter.
The Holocaust, the systematic killing of mainly Jews, but also Gypsies, Poles, and other groups in Europe during World War II that killed 11,000,000 people in all, known in Hebrew as Shoah. Since the 1970s, this has been the main meaning of the English term. See also Names of the Holocaust.
