The political and intellectual tradition that takes the subordination of women to be a structured historical fact, not a natural one, and works to undo it. Feminism is older than the word — Mary Wollstonecraft's A Vindication of the Rights of Woman (1792) is the canonical early text in English — but the term and the organized movements it names are 19th- and 20th-century developments.
The customary periodization speaks of three or four "waves." First-wave feminism (mid-19th century to roughly 1920) fought for legal personhood — property rights, divorce reform, the suffrage. Second-wave feminism (1960s–80s) extended the fight to workplace equality, reproductive rights, sexual violence, the gendered division of domestic and care work, and the cultural construction of femininity itself; Simone de Beauvoir's The Second Sex (1949) provided much of its theoretical groundwork, and Betty Friedan, Kate Millett, Adrienne Rich, Audre Lorde, and the Combahee River Collective shaped its practice. Third-wave and contemporary feminism (1990s–) absorbed the intersectional critique that earlier waves had centered the experience of white middle-class Western women, and made race, class, sexuality, disability, and global-South perspectives central to the analysis. Black feminist thought (bell hooks, Pauli Murray, the Combahee River Collective) was generative throughout, not only in the third wave.
Feminism contains real internal disagreements — liberal vs. socialist vs. radical vs. cultural vs. eco-feminism; arguments over sex work, gender identity, and the relationship to religious tradition. What unifies the tradition is the analytical insistence that gender hierarchy is a political system, and the practical insistence that it can be changed.