Existentialism is a diverse and non-homogeneous stream of thought that has expressed itself in various human cultural and social spheres, including philosophy, literature, art, and custom, which, in the most common sense of the term, affirms the intrinsic value of individual and collective human existence as the core or hinge of reflection, as opposed to other more totalizing, absolutist philosophical currents and principles. It arose between the 18th and 19th centuries, found wide development in the 20th century, and spread and established itself mainly between the late 1920s and the 1950s.
Especially since the 1930s, following the rediscovery of Kierkegaard, the authors and philosophies of existentialism insist on the specific value of the individual and its precarious and finite character; on the meaninglessness, absurdity, and emptiness that characterize the condition of modern man; as well as on the “loneliness in the face of death” in a world that has become either completely alien or hostile.
Idealism, positivism, and rationalism may be far removed from existentialist conceptions. In some existentialists we find religious accents, in others a humanistic and secular character, either pessimistic or optimistic. It influences numerous other parallel and successive philosophies “without ever becoming radical irrationalism”.
Depending on the definition given to the “movement,” a philosopher or philosophical trend may or may not be considered an expression of existentialism. This explains why some of the philosophers considered to be among the major exponents of existentialism (such as Heidegger and Jaspers) have rejected the label, which has instead been taken up as a banner by others, such as Jean-Paul Sartre and Simone de Beauvoir. It was Sartre, in particular, who made the term famous in the philosophical lexicon and in the popular sense with his lecture Existentialism is a Humanism.