I would understand reverse sexism to mean consciously giving preferential treatment to women, all other things being equal (because sexism, bizarrly, means discrimination against women, not just discrimination based on sex/gender). But, of course many versed in feminist theory would disagree with such definition.

Discrimination benefiting historically under represented or discriminated against groups.

Sexism and racism in favor of a group which is usually hurt by sexism or racism. Or equivalently, sexism and racism against a group which usually benefits from sexism and racism.