The word "anti-Semitism" was coined in Germany in 1879 by Wilhelm Marr
as a more euphonious way of saying "Judenhass" (Jew-hatred), and has
always meant exactly that. Its antonym, "Semitism" connoted a positive
attitude toward the Jewish people. The word has become too sanitized
and too easily misunderstood, which is exactly what Marr tried to
accomplish with the word's creation.
According to a (now discredited) nineteenth century theory that held
that racial groups and linguistic groups coincide, Semites are natives
of a group of Middle Eastern nations that are closely related in
ethnicity, culture and language. Under this theory, the modern day
Semites would be the Jews and Arabs.
...Given that the theory of "semites" and non-"semites" is now
discredited, the preferred term to use is "Antisemitism", which has a
general connotation of "anti-Jewish". When written in this fashion, it
helps to eliminate the confusion with the discredited theory. (The use
of the non-hyphenated form is a suggestion of the distinguished
historian James Parkes). Emil Fackenheim, the Jewish philosopher, has
also adopted this spelling, explaining "... the spelling ought to be
antisemitism without the hyphen, dispelling the notion that there is
an entity 'Semitism' which 'anti-Semitism' opposes" (Emil Fackenheim,
"Post-Holocaust Anti-Jewishness, Jewish Identity and the Centrality of
Israel," in World Jewry and the State of Israel, ed. Moshe Davis, p.
11, n. 2).