Will-o'-the-wisp (noun) is a phosphorescent light that appears in the night over marshes and is thought to be due to the combustion of gas from decomposed organic matter. In other words, a ghostly light in a swamp. Another name for it is ignis fatuus. More metaphorically speaking, will-o'-the-wisp can refer to a goal or a person difficult to reach or catch. According to Oxford Dictionaries, this is a 17th century word originally known as "will with the wisp", the wisp being a lighted torch. This always puts to mind Tolkein's Dead Marshes from The Two Towers, as Frodo and Sam follow Gollum through the ghostly lights passed dead things in the water. Freakiest passage ever. Freakiest movie scene as well.