The Collaborative International Dictionary of English v.0.48:

Jack-o'-lantern \Jack"-o'-lan`tern\, Jack-with-a-lantern
\Jack"-with-a-lan`tern\, n.
   1. (Biol.) A large orange-colored luminescent mushroom,
      Clitocybe illudens, also classified as Omphalotus
      olearius. It is poisonous and is sometimes found on
      hardwood tree stumps. [WordNet sense 1]

   Syn: jack-a-lantern, Clitocybe illudens.
        [WordNet 1.5 +PJC]

   2. a pale light sometimes seen at night over marshy ground;
      an ignis fatuus; a will-o'-the-wisp. [WordNet sense
      2]"[Newspaper speculations] supplying so many more
      jack-o'-lanterns to the future historian." --Lowell.

   Syn: friar's lantern, ignis fatuus, will-o'-the-wisp.
        [WordNet 1.5]

   3. A lantern carved from a hollowed-out pumpkin, with holes
      cut in the rind and so shaped that when it is illuminated
      by a candle inside, the features of a human face, cat's
      face, etc. appear in a glowing yellow color. It is used
      mostly as a decoration at Halloween.
      [WordNet 1.5 +PJC]