The term “genocide” was adopted in the immediate aftermath of World War II out of the need to designate, to name, the most horrendous crime perpetrated by the Nazi regime: the systematic, state-run murder of the European Jews. The word itself means “murder of a people,” and while the act of genocide was not invented in the twentieth century - forms of genocide have occurred since the ancient world - never before had a government carried out a genocide that was as far-reaching, as bureaucratically-managed, or as focused as the Holocaust. While much of the Holocaust took the form of blood-soaked massacres, akin to the slaughter of the Armenians by the inchoate state of Turkey in the early 1920s or the various mass killings of Native Americans in the long, bloody colonization of the Americas by Europeans, the Holocaust was also distinct from other genocides in that much of it was industrialized: run on timetables, with the killing occurring in gas chambers built by Nazi agents or private firms contracted to do the work. In short, the Holocaust was a distinctly and horrifyingly modern genocide.
World War II would “just” be the story of a horrendously costly war if not for the Holocaust. The term itself refers to early Jewish rituals of sacrifice by fire, in which offerings were made to God and burned in the ancient Temple of Solomon (long since destroyed by the Romans) in Jerusalem. Today, the term is mostly used in the United States; the rest of the world largely uses the term Shoah, which means "catastrophe" in Hebrew. Its core definition is simple: the ideologically-motivated, brutal murder of approximately 6,000,000 Jews by the Nazi regime, representing two-thirds of Europe’s Jewish population at the time, and one-third of the entire global Jewish population. Thus, in addition to its modern character, the Holocaust stands out among the history of genocides for its shocking “success” from the perspective of the Nazi leadership: they set out to kill every Jew, theoretically in the entire world, and were horrifyingly successful at doing so in a very short time period.
In addition to the murder of the Jews, millions more were killed by the Nazis in the name of their ideology. While estimates vary, at least 250,000 Romani (“Gypsies”) were murdered. At least 6,000 male homosexuals were murdered. Many thousands of ideological “enemies,” from Jehovah’s Witnesses to various kinds of political leftists, were murdered as well. In addition, while not normally considered part of the Holocaust per se, almost 20,000,000 civilians in the Slavic nations - Poles and Russians especially - were murdered by the Nazis in large part because of Nazi racial ideology. Slavs too were “racial inferiors” and “subhumans” according to the Nazi racial hierarchy, and thus civilian populations in the Slavic countries were either killed outright or subjected to treatment tantamount to murder. Thus, while the Holocaust is, and must be, defined primarily as the genocide of the European Jews by the Nazis, it is still appropriate to consider the other victims of Nazi ideology as an aspect of Nazi mass murder as a whole.
Thumbanil: "Arbeit macht frei" is a German phrase meaning "work sets you free". The slogan is known for appearing on the entrance of Auschwitz and other Nazi concentration camps. This is the Arbeit Macht Frei gate in KZ Sachsenhausen, Berlin. (CC BY-SA 1.0 Generic; jpatokal via Wikipedia).