Existentialism is a philosophical movement that emphasizes individual existence, freedom, and choice. It asserts that individuals are responsible for creating their own meaning and values in an inherently meaningless or absurd universe. Key figures include Jean-Paul Sartre, Simone de Beauvoir, and Albert Camus, who explored themes of alienation, anxiety, and the human condition. Existentialist thought often encourages personal authenticity and the courage to confront life's inherent uncertainties.