Existentialism is generally considered to be the philosophical and cultural movement which holds that the starting point of philosophical thinking must be the individual and the experiences of the individual, that moral thinking and scientific thinking together do not suffice to understand human existence, and, therefore, that a further set of categories, governed by the norm of authenticity, is necessary to understand human existence. (Authenticity, in the context of existentialism, is being true to one's own personality, spirit, or character.)

Existentialism began in the mid-19th century as a reaction against then-dominant systematic philosophies, with Søren Kierkegaard generally considered to be the first existentialist philosopher. Opposed to Hegelianism and Kantianism, Kierkegaard posited that it is the individual who is solely responsible for giving meaning to life and for living life passionately and sincerely, even in view of its many obstacles and distractions.

Existentialism became popular in the years following World War II and influenced a range of disciplines besides philosophy, including theology, drama, art, literature, and psychology.