On the existential view, to understand what a human being is it is not enough to know all the truths that natural science—including the science of psychology—could tell us. . . . neither scientific nor moral inquiry can fully capture what it is that makes me myself, my “ownmost” self.

Without denying the validity of scientific categories (governed by the norm of truth) or moral categories (governed by norms of the good and the right), “existentialism” may be defined as the philosophical theory which holds that a further set of categories, governed by the norm of authenticity, is necessary to grasp human existence.

To approach existentialism in this categorial way may seem to conceal what is often taken to be its “heart” (Kaufmann 1968:12), namely, its character as a gesture of protest against academic philosophy, its anti-system sensibility, its flight from the “iron cage” of reason. But while it is true that the major existential philosophers wrote with a passion and urgency rather uncommon in our own time, and while the idea that philosophy cannot be practiced in the disinterested manner of an objective science is indeed central to existentialism, it is equally true that all the themes popularly associated with existentialism—dread, boredom, alienation, the absurd, freedom, commitment, nothingness, and so on—find their philosophical significance in the context of the search for a new categorial framework, together with its governing norm.

(Via Stanford Encyclopedia of Philosophy)