The Collaborative International Dictionary of English v.0.48:

Ignis fatuus \Ig"nis fat"u*us\; pl. Ignes fatui. [L. ignis
   fire + fatuus foolish. So called in allusion to its tendency
   to mislead travelers.]
   1. A phosphorescent light that appears, in the night, over
      marshy ground, supposed to be occasioned by the
      decomposition of animal or vegetable substances, or by
      some inflammable gas; -- popularly called also
      Will-with-the-wisp, or Will-o'-the-wisp, and
      Jack-with-a-lantern, or Jack-o'-lantern. It is thought
      by some to be caused by phosphine, PH3, a sponaneously
      combustible gas.
      [1913 Webster + PJC]

            Will o'the Wisp -- which also rejoices in the names
            of Ignis Fatuus or Jack o'Lantern -- is not, as some
            of you may think, a cartoon character. In mediaeval
            times this chemical phenomenon struck terror into
            travellers and, very likely, lured some of them to
            their deaths in a stinking and marshy grave.
            I have never seen this Will o'the Wisp; nor am I
            likely to do so. It is a flickering flame seen over
            marshes; marshes are not now common in London, nor
            indeed anywhere else in Britain. In any case the
            ephemeral nature of the phenomenon and the enormous
            amount of ambient light [ldqo]pollution[rdqo] found
            in most areas means that most of us will never see
            it.
            What is this Will o'the Wisp? Popular chemical lore
            has it that it is marsh gas, or methane, which
            catches fire when it hits the air because of the
            presence of either phosphine (PH3) or diphosphine
            (P2H4) in the gas, both of which are spontaneously
            flammable in air. Methane is certainly formed in
            marshes, and bubbles up if the mud is disturbed in a
            pond, say. It is the same reaction that enables
            organic materials to produce biogas, methane from
            the decomposition of sewage, which can be profitably
            used. But is it this that is burning in Will o'the
            Wisp?
            Almost certainly not. At this point I will say that
            I have thought for some years off and on as to how
            one might set up an experiment to test the
            hypotheses, since the sporadic and rare nature of
            the natural version renders its investigation a
            highly intractable problem. However: the combustion
            of methane under the conditions in a marsh would
            give a yellow flame, and heat.
            Will o'the Wisp is not like this, so it is said.
            Firstly the flame is bluish, not yellow, and it is
            said to be a cold flame. The colour and the
            temperature suggests some sort of phosphorescence;
            since organic material contain phosphorus, the
            production of phosphine or diphosphine is scarcely
            impossible, and maybe it does oxidise via a mainly
            chemiluminescent reaction. The exact nature of the
            Will o'the Wisp reaction nevertheless remains, to me
            at any rate, a mystery. Similar phenomena have been
            reported in graveyards and are known as corpse
            candles. If anyone knows anything more, I would love
            to hear of it. A warning that if you look for it on
            the Web, you will get a great deal of bizarre stuff.
            You will also get the delightful picture from a
            Canadian artist which decorates the top of this page
            (http://www.rod.beavon.clara.net/willo.htm), and a
            couple of poems at least. One is also by a Canadian,
            Annie Campbell Huestis, the other by the prolific
            fantasy poet Walter de la Mare.
            The preparation of phosphine in the laboratory (by
            the teacher!) is fun, and perfectly safe in a fume
            cupboard. White phosphorus is boiled with aqueous
            sodium hydroxide solution in an apparatus from which
            all air must have been removed by purging with, say,
            natural gas. The phosphine will form marvellous
            smoke rings if allowed to bubble up through water in
            a pneumatic trough. This is an experiment for the
            teacher, needless to say. The experiment is
            described in Partington J.R., [ldqo]A Textbook of
            Inorganic Chemistry[rdqo], 6th ed, Macmillan 1957, p
            572. (So, inter alia, is a great deal of other
            interesting chemistry.)
      Dr. Rod Beavon
      17 Dean's Yard London SW1P 3PB
      e-mail: rod.beavon@westminster.org.uk
      [PJC]

   2. Fig.: A misleading influence; a decoy.
      [1913 Webster]

            Scared and guided by the ignis fatuus of popular
            superstition.                         --Jer. Taylor.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ignis fatuus
    n 1: a pale light sometimes seen at night over marshy ground
         [syn: friar's lantern, ignis fatuus, jack-o'-lantern,
         will-o'-the-wisp]
    2: an illusion that misleads [syn: will-o'-the-wisp, ignis
       fatuus]

Moby Thesaurus II by Grady Ward, 1.0:

94 Moby Thesaurus words for "ignis fatuus":
   airy nothing, autism, autoluminescence, backfire, balefire, beacon,
   beacon fire, bioluminescence, blaze, bonfire, bubble, burning ghat,
   campfire, cathode luminescence, cheerful fire, chemicoluminescence,
   chemiluminescence, chimera, combustion, conflagration, corposant,
   cozy fire, crackling fire, crematory, crystalloluminescence,
   daydream, death fire, deception, deluded belief, delusion, dereism,
   double corposant, dream, dream vision, dreamland, dreamworld,
   electroluminescence, false belief, fata morgana, fen fire, fire,
   flame, flashing point, flicker, flickering flame, fluorescence,
   forest fire, fox fire, funeral pyre, hallucination, ignition,
   illusion, ingle, lambent flame, luciferase, luciferin,
   luminescence, luminophor, marshfire, mirage, misbelief,
   misconception, noctiluscence, open fire, phantasm, phosphor,
   phosphorescence, photoluminescence, pipe dream, prairie fire, pyre,
   radioluminescence, raging fire, sea of flames, self-deceit,
   self-deception, self-delusion, sheet of fire, signal beacon,
   smudge fire, thermoluminescence, three-alarm fire,
   tribofluorescence, triboluminescence, tribophosphorescence, trick,
   trip, two-alarm fire, vapor, watch fire, wildfire, wisp,
   witch fire, wrong impression