Existentialism is a philosophy which stresses the importance of human experience, and says that everyone is responsible for the results of their own actions.
[technical]
existentialism in British English
(ˌɛɡzɪˈstɛnʃəˌlɪzəm)
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
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 essence and holds that human beings are totally free and responsible for their acts and thatthis responsibility is the source of their feelings of dread and anguish