A system of social relations in which men, as a group, exercise dominance over women, as a group — through law, economic structures, sexuality, violence, and the everyday practices of family, work, and culture. The term was given its contemporary analytical meaning by second-wave feminists in the late 1960s and 1970s (Kate Millett's Sexual Politics, 1970, is a landmark) who argued, building on Simone de Beauvoir, that relations between the sexes are a political system in the strict sense: organized, historical, sustained by specific institutions and ideologies, and therefore analyzable and changeable.
Three features made the concept theoretically powerful. First, it named a structure rather than a collection of individual prejudices — shifting the analysis from "some men are sexist" to "this society is organized along these lines." Second, it linked apparently disparate phenomena (wage gaps, domestic violence, beauty norms, legal doctrines around rape and consent, the division of care work) as expressions of a common underlying logic. Third, it made comparative analysis possible: patriarchies vary across cultures and periods, so their specific mechanisms can be studied rather than treated as natural.
Within feminist theory the concept has been extensively refined and contested. Early uses were sometimes criticized as universalizing the experience of white middle-class Western women; intersectional analysis insists that patriarchy always operates together with race, class, sexuality, and colonial relations, never as a pure structure in isolation. Marxist feminists have debated the relationship between patriarchy and capitalism — separate systems, welded system, or two faces of one? Some contemporary feminists prefer more specific terms ("male dominance," "gender regimes," "sex/gender systems") to mark these analytical distinctions.