By Jonathan Marcus
Diplomatic correspondent, BBC News
The Holocaust is the name given to the Nazi regime's murder of six million European Jews during World War II.
More than one million people died at the Auschwitz death camp
Other groups like gypsies and homosexuals were also killed in large numbers, but the term "holocaust" is generally seen as marking out the Jewish experience - an effort by a modern state to remove a whole ethnic group from the face of the earth.
The Nazis perfected a system of industrialised killing, herding many of their victims into gas chambers, after which their bodies were burnt in crematoria.
Throughout much of the world - at least in respectable circles - all this is regarded as incontrovertible fact.
Some of the camps, like Auschwitz, are still there. There is the testimony of survivors, and even of some of their guards. There is the evidence from the war crimes trials held in Nuremburg. And a vast body of academic literature has developed, chronicling the Holocaust in extraordinary detail.
Ultimately, there is the silent witness of the victims themselves - the millions who disappeared into the "night and fog" of what the Nazis themselves termed "the final solution" to the Jewish question.
Thus, the official Iranian conference to review the Holocaust flies in the face of established historical fact.
True, there are those in America and Europe, on the fringes of academic and political life, who deny that these events happened. Some of these people have been invited to Tehran to take part in the conference.
But, ultimately, this conference is not about history but present-day politics.
Iran wants to de-legitimise its great enemy - Israel - and clearly regards the Holocaust as one of the key factors that led to the creation of a Jewish state.