Cornelius Castoriadis (Greek: Κορνήλιος Καστοριάδης) was a Greek philosopher, social critic, economist, psychoanalyst, author of The Imaginary Institution of Society, and co-founder of the Socialisme ou Barbarie group.

Edgar Morin proposed that Castoriadis' work will be remembered for its remarkable continuity and coherence as well as for its extraordinary breadth which was "encyclopaedic" in the original Greek sense, for it offered us a "paideia," or education, that brought full circle our cycle of otherwise compartmentalized knowledge in the arts and sciences. Castoriadis wrote essays on mathematics, physics, biology, anthropology, psychoanalysis, linguistics, society, economics, politics, philosophy, and art.

One of Castoriadis' many important contributions to social theory was the idea that social change involves radical discontinuities that cannot be understood in terms of any determinate causes or presented as a sequence of events. Change emerges through the social imaginary without determinations, but in order to be socially recognized must be instituted as revolution. Any knowledge of society and social change “can exist only by referring to, or by positing, singular entities…which figure and presentify social imaginary significations.”

Castoriadis used traditional terms as much as possible, though consistently redefining them. Further, some of his terminology changed throughout the later part of his career, with the terms gaining greater consistency but breaking from their traditional meaning (neologisms). When reading Castoriadis, it is helpful to understand what he means by the terms he uses, since he does not redefine the terms in every piece where he employs them.

Castoriadis has influenced European (especially continental) thought in important ways. His interventions in sociological and political theory have resulted in some of the most well-known writing to emerge from the continent (especially in the figure of Jürgen Habermas, who often can be seen to be writing against Castoriadis). Hans Joas published a number of articles in American journals in order to highlight the importance of Castoriadis' work to a North American sociological audience, and the enduring importance of Johann P. Arnason, both for his critical engagement with Castoriadis' thought, but also for his sustained efforts to introduce Castoriadis' thought to the English speaking public (especially during his editorship of the journal Thesis Eleven) must also be noted. In the last few years, there has been growing interest in Castoriadis’s thought, including the publication of two monographs authored by Arnason's former students: Jeff Klooger's Castoriadis: Psyche, Society, Autonomy (Brill), and Suzi Adams's Castoriadis's Ontology: Being and Creation (Fordham University Press).