"Holocaust denial in its various forms is an expression of antisemitism."
Holocaust denial is discourse and propaganda that deny the historical reality and the extent of the extermination of the Jews by the Nazis and their accomplices during World War II, known as the Holocaust or the Shoah. Holocaust denial refers specifically to any attempt to claim that the Holocaust/Shoah did not take place. Holocaust denial may include publicly denying or calling into doubt the use of principal mechanisms of destruction (such as gas chambers, mass shooting, starvation and torture) or the intentionality of the genocide of the Jewish people. Holocaust denial in its various forms is an expression of antisemitism. The attempt to deny the genocide of the Jews is an effort to exonerate National Socialism and antisemitism from guilt or responsibility in the genocide of the Jewish people. -- International Holocaust Remembrance Alliance Working Definition of Holocaust Denial and Distortion
This three-minute video from the United States Holocaust Memorial Museum explains Holocaust denial and the different forms it takes.