> Well, I don't know if "sexism" has an "objective" definition, but yes, words are usually best defined by experts on the subjects. Just as gravity is best defined by physicists, sexism is best defined by feminists, who have studied sexism for the past few decades.
Feminism is an ideology, not a field of study (well, one can study "feminism" in that you can study what feminists believe and do, but that's not the subject here.)
Yes, feminism is an ideology, but one that is universally shared among those who study gender and sexism, just as anti-racism is an ideology shared by those who study racism. You can dismiss it based on ignorance, or try to listen for a second and might learn something.
Feminism is an ideology, not a field of study (well, one can study "feminism" in that you can study what feminists believe and do, but that's not the subject here.)