The noun holocaust means "total destruction." However, this word has become inextricably connected with World War II and the mass murder of Jews by the Nazis (in this usage, Holocaust is capitalized).
When holocaust entered the English language in the thirteenth century, it referred to burnt offerings described in the Old Testament. The original Greek word meant "burnt whole" (note the relationship to the word caustic). Since at least 1942, though, the word is most often used to describe the extermination of Jews by the Nazis. Some people object to the religious connotations of the word, though, preferring to use the Hebrew Shoah (catastrophe) instead.