In its narrow meaning, patriarchy refers to the system, historically derived from Greek and Roman law, in which the male head of the household had absolute legal and economic power over his dependent female and male family members. Patriarchy in its wider definition means the manifestation and institutionalization of male dominance over women and children in the family and the extension of male dominance over women in society in general. It implies that men hold power in all the important institutions of society and that women are deprived of access to such power. It does not imply that women are either totally powerless or totally deprived of rights, influence, and resources. Women are a Sex.
Women are a separate group due to their biological distinctiveness.
The merit of using the term is that it clearly defines women, not as a subgroup or a minority group, but as half of the whole. Men are the only other sex. Obviously, we are here not referring to sexual activity, but to a biological given. Sexism and patriarchy mutually reinforce one another. Gerda Lerner, The Creation of Patriarchy Patriarchal attitudes.