Dream vs Nightmare?

So as a community we have concluded that the great ones reside in dreams/nightmare (4th dimension) however I don’t know what defines this plane as a dream or nightmare. Is a nightmare labelled that purely because of the hostility within it? If so why is the hunters dream still labeled a dream in the endgame. Dreams have always fascinated me in bloodborne but I have never really understood what / how. Another thing I can’t seem to understand is, why is the hunters dream, a dream? Germahn explicitly calls it a nightmare several times, is this because he hates the dream, or is it secretly a nightmare, something completely else that all the hunters never suspected? If anyone knows the answer or even has a theory on it, please let me know I can’t find any other answers online.