Definition of: existentialism

(eg′zis·ten′shəl·iz′əm) noun
1. A movement in 20th century philosophy, influenced in its development by Kierkegaard and Nietzsche and popularized in France by Sartre, emphasizing the active participation of the will, rather than the reason, in confronting the problems of a non–moral or absurd universe. Man is defined in existentialism as the sum total of his acts rather than his intentions or potentialities, and exists in order to will himself to act.
2. A cult of nihilism and pessimism popularized in France after World War II, supposedly based on the doctrines of Sartre and other existentialist writers.
—ex′is·ten′tial·ist adjective & noun