What is feminism? I like the definition “Feminism is the radical idea that women are people”.
Feminism can also be defined via its antagonist, by what it tries to fight: sexism. So what is sexism?
Sexism is the systematic structural society-wide oppression, disadvantaging and devaluing of women and devaluing of everything feminine, i.e. everything that is considered typically female in the society.
On a more abstract level it is the discrimination and domination of one gender (in societies with three or more traditional genders usually: of all genders but one) by the other one. In the few remaining matriarchic societies sexism is indeed the discrimination of men, and there there could be masculist movements that fight the discrimination of men.
In most societies that currently exist on this planet, e.g. in all of those in Europe and North America (as far as I am aware including Native Americans) men are the privileged and dominant, i.e. systematically advantaged group.
Feminism fights for equality between women and men. As women are the marginalized group, it fights for women. This is why it’s called feminism, and there is no reason to rename it to e.g. equalism.