Radical feminism is the current that refused to treat sex inequality as an unfinished piece of liberal business. Its claim is that patriarchy is a system in its own right — historically prior to capitalism, not reducible to it, and reproduced through institutions that formal equality leaves entirely intact.
The word radical means to the root. Where liberal feminism sought access — the vote, the professions, equal pay — radical feminists argued that a woman admitted to institutions built around a male life was being offered a place in someone else's arrangement. Millett's Sexual Politics (1970) made the case by reading power relations in the household and the bedroom as political rather than private, giving the movement its central slogan: the personal is political. Firestone, in The Dialectic of Sex (1970), took the argument to its limit, locating the root of subordination in reproduction itself and looking to technology to break the biological arrangement on which she thought every other hierarchy was modeled.
MacKinnon turned it into law. Her argument is that inequality between the sexes is not chiefly a matter of irrational prejudice but of eroticized dominance, and that pornography and sexual harassment are practices of subordination rather than instances of expression. She built the legal category of sexual harassment more or less single-handedly, which makes her among the most practically consequential legal theorists of the century, and her attempt with Andrea Dworkin to make pornography actionable as a civil rights violation split feminism in the sex wars of the 1980s.
Its own daughters were its sharpest critics. Black feminists including bell hooks and Kimberlé Crenshaw argued that a movement theorizing "women" as a unified class had generalized the position of white middle-class women and called it universal — the objection that produced intersectionality.