In your opinion, can non-Jews tell Jews what constitutes as antisemitism? ?

Answer 1

Christians are forever being told by non Christians what constitutes persecution. They are also continuously mocked and ridiculed across the entertainment industry and arts. 

Refer to: Jesus on the Cross in a huge container of urine, displayed in a New York art gallery. 

Try doing that with the Star of David or Menorah.  

https://en.wikipedia.org/wiki/Piss_Christ

Answer 2

Your question is nonsensical.

Well defined concepts have well defined definitions. Antisemitism is well defined. Thus, something is antisemitic because it meets such a definition.

In general, I don't see a disparity between "how Jews define antisemitism" and "how non Jews define antisemitism". In fact, the types of people who seek to REdefine the word are antisemites who wish to avoid the stigma of their own noxious views.

Answer 3

Yes, if they themselves have lived experience of racism, because they will be able to draw an analogy.  Anti-Semitism is a form of racism, and whereas the specifics may be different, they generally are for each ethnicity.