Dasein is a philosophical concept. You've seen it in English because it's been codified (as much of Freud should have been) in German - there's really no good translation. I believe it was Heidegger who coined the term in his treatise, Being and Time. Dasein ("being there") is the condition of the concretely existing human vis-a-vis his world. It's complicated. It's existentialist. I don't much get it myself.