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It is hostility towards or discrimination against Jews.
Antisemitism (alternatively spelled anti-semitism or anti-Semitism, also known as Judeophobia) is prejudice against and hostility toward Jews as a religious, racial, or ethnic group. While the term's etymology might suggest that antisemitism is directed against all Semitic peoples, since its creation in c. 1879 it has been used exclusively to refer to hostility towards Jews. The meaning of a word is its use, not its etymology. (This applies particularly to invented words like this). There are no known cases of the use of the word antisemitism in the sense of anti-Arab.
Antisemitism is also a political ideology that ascribes all the misfortunes of a country or of even of the whole world to an imaginary "Jewish conspiracy". As such, it is a conspiracy theory.
Antisemitism is racist (rather than religious) hatred of Jews. Some scholars draw a distinction between anti-Judaism (hostility to the Jewish religion and it followers) and antisemitism (which is the much more recent political and racial conspiracy theory). Obviously, the latter (or more recent) draws on the former (earlier).
People who criticize Jewish people or the Jewish religion are called anti-Semites.
