Ethereally is ethereal turned into an adverb: in that delicate, light, not-quite-real way. Someone sings ethereally; light falls ethereally; a figure appears ethereally in fog. The meaning does not change — still thin, fine, and almost otherworldly — but now it tells you how something happens or how it seems, not what kind of thing it is. It is even more literary than ethereal. You will meet it in reviews and stories more than in everyday chat. If you want to name the quality, use the adjective (an ethereal voice). If you want to say how the singing felt, use this word (she sang ethereally).

空靈地