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
see also:
clitocybe illudens omphalotus olearius ignis fatuus