@Victor Eremita,
Sarte's subtitle for Being and Nothingness is "An Essay on Phenomenological Ontology," yet he himself coined the term Existentialism. The alliance between Existentialist and Phenomenological thinking almost seems to blur any distinction for the the purposes of this thread.
We should probably acknowledge that none of the philosophers would be satisfied or happy being called Existentialists or even associated with
any school of thought, and simply accept the term's utility in discussing a group of often independent-minded philosophers with family resemblances.
We may wish to say undogmatically and strictly tentatively that what seems to be common amongst these thinkers is that the problem of being and of knowledge cannot be made a subject of objective inquiry, but must begin with the Self's reflection on his own particular existence in time and space; that is to say that in some sense, existence, or "presence" is basic to any understanding of the world from an ontological, ethical and an epistemological horizon. Sartre, in his famous essay "Existentialism is a Humanism," writes that what unites the atheistic and Christian existentialists is a common assumption that "existence precedes essence, or if you prefer, that subjectivity must be the starting point" of philosophical thinking.
We seem to have some more or less clear historical groupings:
I. Precursors---e.g. SK or FWN
II. Mid-Century Existentialists---Jaspers, Sartre, Heidegger
III. Late Existentialists---Schultz, Foucault
In III, one can include many of the contemporary Hermeneutical thinkers as well.