In English translations, the word rendered "existentiell" was, with the philosophical meaning discussed in this article, first used by Martin Heidegger.
Heidegger distinguishes between his two terms "existential" and "existentiell" in the Introduction to Being and Time. In the work Being and Time, the word existentiell is used to describe an ontic understanding of beings in the world. An existentiell understanding addresses the facts about things in the context of the world, in terms of their existence, but differs from the ontological understanding that however vague is a necessary precondition for ontic understanding. The latter is reached by going about our daily business, interacting with things in the world, whereas existential understanding is theoretical and ontological in character. Heidegger claims that his examination of Dasein (human being) is an existential analysis. However, this is not to disparage existentiell understanding--Heidegger argues that any authentic potentialities of Dasein (the human being in this case) brought out in the existential analysis must be realized in existentiell understanding; i.e. it is the role of the existential analysis to function as a hermeneutic, to explicitly question Being and to interpret its structure, but Da-sein is not acting authentically until those interpretations are realized in Da-sein's ontic life. Further, it is impossible to draw an absolute categorical line between the two, in the sense that existential analysis is itself one of da-sein's possible ways of being, it cannot be simply extracted from and opposed to everyday life. It arises or is chosen just as any of da-sein's other existentiell activities are. Existential analysis is, therefore, existentiell, but in a unique sense, qualitatively distinct from non-theoretical, non-phenomenological behavior. On the other hand, da-sein's "everydayness," or existentiell living, always already discloses an understanding of its existential structures as a condition of its very being and acting.
Though it is not commonly used in philosophy outside of discussions of Heidegger's seminal work Being and Time, it is important to understand Heidegger's definition of the term if one wishes to study Being and Time.
Dasein always understands itself in terms of its existence, in terms of its possibility to be itself or not be itself. Dasein has either chosen these possibilities itself, stumbled upon them, or already grown up in them. Existence is decided only by each Dasein itself in the manner of seizing upon or neglecting such possibilities. We come to terms with the question of existence always only through existence itself. We shall call this kind of understanding of itself existentiell understanding. The question of existence is an ontic "affair" of Dasein. For this the theoretical perspicuity of the ontological structure of existence is not necessary. The question of structure aims at the analysis of what constitutes existence. We shall call the coherence of these structures existentiality. Its analysis does not have the character of an existentiell understanding but rather an existential one. The task of an existential analysis of Dasein is prescribed with regard to its possibility and necessity in the ontic constitution of Dasein.
Heidegger did not coin the term "existentiell". The common German word "existenziell" is usually translated into English as "existential". However, in Heidegger's works, he coined the German word "existenzial", giving it a meaning distinct from the common German word "existenziell". In English translations of Heidegger, then, the German "existenziell" is transliterated as "existentiell" in English, and the German word "existenzial" is transliterated as "existential", each word having its own technical meaning specific to Heidegger.
- Martin Heidegger, Being and Time, Introduction, Section 4. Basic Works pg. 52, HarperCollins Publishers, 1977