>In the case of "sexism", when we are talking about it here we are usually talking about it from within the disciplines that actually deal with it

No, we are not. That is the entire point. We are not in a women's studies department. We are not women's studies majors. We are not talking about women's studies. So the terminology of women's studies is not relevant.

>Within all of these fields, sexism is a technical term that means (with slight variations between fields) exactly what steveklabnik above summarised it as.

No, it is not. History and philosophy do not use sexism that way. Only a minority of sociologists do. The only example you listed that is actually correct is women's studies.