A phosphorescent light seen over marshy ground at night, caused by spontaneous combustion of gases emitted by decomposing organic matter.
日本語で訳すると鬼火で死体が分解される過程でリン酸中のリンが発光する現象だったと推測される。。
Funny thing is that a synonym is foxfire (not Firefox), especially for luminescence produced by fungi.
Will-o'-the wisp has the same meaning.
http://en.wikipedia.org/wiki/Will-o%27-the-wisp
