Existentialism is the term used to refer to a number of related philosophical points of view which began with Kierkegaard as a reaction to the abstract rationalism of Hegel. Existentialist saw the term "existentialism" as essentially meaningless, but still it does refer to certain conventions and views that link different thinkers. Existentialism first declares that existence is prior to essence. This seems a simple statement but has been difficult to explain in the different philosophical statements made by existentialists. Different philosophers have emphasized different consequences of this view.
Friedrich Nietzsche is not strictly speaking an existentialist, though some of his ideas are pre-existentialist in tone and meaning and influenced those who would follow. A superficial reading of Nietzsche links him with Kierkegaard, but he is in a different tradition. Nietzsche repudiated Christianity because he saw it as the enemy of reason. Nietzsche was not considered an existentialist until he was re-examined after World War II:
Judged by our initial criteria, Nietzsche might well be called an existentialist. The refusal to belong to any school of thought, the repudiation of the adequacy of any body of beliefs whatever, opposition to philosophic systems, and a marked dissatisfaction with traditional philosophy as superficial, academic, and remote from life--all this is eminently characteristic of Nietzsche no less than of Kierkegaard, Jaspers, or Heidegger.
Nietzsche is cited by Camus and Sartre and has been absorbed by Jaspers and Heidegger.
Nietzsche in Beyond Good and Evil comments on the tendency of psychologists to place the instinct for self-preservation in the role of the cardinal instinct of the organic being, but Nietzsche differs in this view and writes:
Psychologists should bethink themselves before putting down the instinct of self-preservation as the cardinal instinct of an organic being. ...