By the definition on "Time and Being", Dasein is a human existence or each of human beings exist as themself. The existence has the superiority of existence or in ontology. Because Dasein owns the problem of the existence of the self itself when it exists, and can understand the self and other existences.
Heidegger intentionally used the word Dasein in order to make a question a whole real existence itself of a self, not an abstract general being or a somebody else’s business.
Originally, the word Dasein is a german word made by Christian Wolff, to translate a latin word existentia. The word means being existence distinguished by essentia (essence). Also nowadays, an ordinary german word Dasein means “being”, “living” or “presence”.
“Da-“ of the word Dasein means "real” also “place” the world opens for us, and the place means the place of Erschlossenheit (disclosure). The characteristics of the existence as a human is it exists in the place. So the word Dasein means the existence as a human, and is used to suggest the specific way of exist of this existence.
References
Gen Kida, Knowledge of Heidegger 88 (Shinshokan, 2002)
Michael Inwood, Heidegger: A Very Short Introduction (Oxford University Press, 1997)
Mark Wrathall, How to Read Heidegger (Granta Books, 2005)