ig'nis-fat'[=u]-us, n. a light which misleads travellers, often seen over marshy places, also called 'Will-o'-the-Wisp:'--pl. IGNES-FATUI (ig'n[=e]z-fat'[=u]-[=i]). [L. ignis, fire, fatuus, foolish.]
n. [Latin] A light that appears in the night over marshy grounds, supposed to be occasioned by the decomposition of animal or vegetable substances, producing phosphoric or other inflammable gas;—Will of the wisp; Jack o' lantern.