Being and Time by Martin Heidegger is, in the words of William Large, "probable one of the most important books written in the twentieth century" (1). It influenced not only existentialism, but the whole of contentential philosophy. One of Heidegger's main concepts is Dasein. Dasein is Heidegger's "word for human existence.... which describes the way in which human beings are rather than what they are" (Large 109-110). I would translated Dasein into English as "human beingness."

Reference

Large, William (2008) Heidegger's Being and Time. Bloomington, IN: Indiana University Press.