What is the definition of antisemitism?

 Anti-Semitism refers to prejudice and persecution against Jews.

Explanation/details:

Anti-Semitism was strong in Europe already in the Middle Ages, when Jews were accused of such things as spreading the plague by poisoning wells, or using the blood of murdered Christians to make the matzah for their Passover rituals.  The term "anti-Semitism" as a description for hostile opposition to the Jewish people was first used by Wilhelm Marr in 1879 in Germany.  Marr supported campaigns against Jews and began using the term "anti-Semitism" as a euphemism for what better might have been called "Jew-hating."

Anti-Semitism throughout Europe in the late 19th and early 20th centuries prompted the Zionist movement.  The Zionist movement was a nationalist cause for Jews to establish a homeland for themselves in the ancient territorial home of the Jewish people, in the region which had become known as Palestine. (That name was based on the old Roman empire name for the "Philistine" region.)

Anti-Semitism saw its strongest expression in the Holocaust -- mass murder of million of Jews carried out by the Nazi regime in Germany during World War II.  

Anti-Semitism continues to be a problem today, as was seen when a man carried out a mass shooting at a Jewish synagogue in Pittsburgh on October 27, 2018, while worshipers were gathered there.

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