A ghost — but a particular, colder kind of ghost, and the word is literary rather than everyday. In its oldest Scottish use, a wraith was the shape of a living person seen when that person was somewhere else, and seeing one meant they were about to die. That warning feeling still clings to the word. In modern writing a wraith is usually a thin, pale, drifting spirit, and writers reach for it when they want something more frightening and less homely than "ghost." It is common in fantasy stories and games, where wraiths are undead things that float and drain life. There is a second, gentler use: you can call a very thin, pale person a wraith, or say someone is "like a wraith," and you can use it for something faint and drifting in the air — a wraith of smoke, a wraith of mist. The adjective is wraithlike. You would not use this word in conversation about a haunted house; you would say ghost. Save wraith for writing, or for when you want the room to feel colder.