The Collaborative International Dictionary of English v.0.48:

Jungle \Jun"gle\ (j[u^][ng]"g'l), n. [Hind. jangal desert,
   forest, jungle; Skr. ja[.n]gala desert.]
   1. A dense growth of brushwood, grasses, reeds, vines, etc.;
      an almost impenetrable thicket of trees, canes, and reedy
      vegetation, as in India, Africa, Australia, and Brazil.

            The jungles of India are of bamboos, canes, and
            other palms, very difficult to penetrate. --Balfour
                                                  (Cyc. of
                                                  India).
      [1913 Webster]

   2. Hence: (Fig.) A place of danger or ruthless competition
      for survival. /'bdIt's a jungle out there./'b8
      [PJC]

   3. Anything which causes confusion or difficulty due to
      intricacy; as, a jungle of environmental regulations.
      --MW10.
      [PJC]

   Jungle bear (Zool.), the aswail or sloth bear.

   Jungle cat (Zool.), the chaus.

   Jungle cock (Zool.), the male of a jungle fowl.

   Jungle fowl. (Zool.)
      (a) Any wild species of the genus Gallus, of which
          several species inhabit India and the adjacent
          islands; as, the fork-tailed jungle fowl (Gallus
          varius) of Java, Gallus Stanleyi of Ceylon, and
          Gallus Bankiva of India.

   Note: The latter, which resembles the domestic gamecock, is
         supposed to be one of the original species from which
         the domestic fowl was derived.
      (b) An Australian grallatorial bird (Megapodius tumulus)
          which is allied to the brush turkey, and, like the
          latter, lays its eggs in mounds of vegetable matter,
          where they are hatched by the heat produced by
          decomposition.
          [1913 Webster]

WordNet (r) 3.0 (2006):

jungle
    n 1: a location marked by an intense competition and struggle
         for survival
    2: a place where hoboes camp [syn: hobo camp, jungle]
    3: an impenetrable equatorial forest

Moby Thesaurus II by Grady Ward, 1.0:

77 Moby Thesaurus words for "jungle":
   Chinese puzzle, Gordian knot, Rube Goldberg contraption,
   afforestation, arboretum, boondocks, bush, bushveld, can of worms,
   chase, climax forest, cloud forest, complex, dendrology, forest,
   forest land, forest preserve, forestry, fringing forest,
   gallery forest, greenwood, hanger, hash, hugger-mugger,
   index forest, jumble, jungles, knot, labyrinth, litter, mash, maze,
   meander, mesh, mess, mishmash, morass, muddle, national forest,
   palmetto barrens, park, park forest, perplex, pine barrens,
   primeval forest, protection forest, rain forest, ravel,
   reforestation, rummage, scramble, scrub, scrubland,
   selection forest, shrubland, silviculture, skein, snafu, snake pit,
   snarl, sprout forest, stand of timber, state forest, tangle,
   tangled skein, timber, timberland, tree veld, virgin forest, web,
   webwork, wheels within wheels, wilderness, wildwood, wood,
   woodland, woods