Simone de Beauvoir was a French writer, philosopher, and feminist activist. She is best known for her groundbreaking work "The Second Sex," which is considered a foundational text in the field of feminist philosophy. De Beauvoir was a key figure in the existentialist movement and had a significant influence on the development of feminist theory. She was also a lifelong partner of the philosopher Jean-Paul Sartre. De Beauvoir's work continues to be influential in discussions of gender, ethics, and existentialism.