Real Dictionary

existentialism
uncountable noun: Existentialism is a philosophy which stresses the importance of human experience, and says that everyone is responsible for the results of their own actions.

existentialism in American English
a philosophical and literary movement, variously religious and atheistic, stemming from Kierkegaard and represented by Sartre, Heidegger, etc.: it is based on the doctrine that concrete, individual existence takes precedence over abstract, conceptual esse
noun: a philosophical attitude associated esp. with Heidegger, Jaspers, Marcel, and Sartre, and opposed to rationalism and empiricism, that stresses the individual's unique position as a self-determining agent responsible for the authenticity of his or her choi

existentialism in British English
noun: a modern philosophical movement stressing the importance of personal experience and responsibility and the demands that they make on the individual, who is seen as a free agent in a deterministic and seemingly meaningless universe