Source : Webster's Revised Unabridged Dictionary (1913)

Hedgehog \Hedge"hog`\, n. (Elec.)
   A variety of transformer with open magnetic circuit, the ends
   of the iron wire core being turned outward and presenting a
   bristling appearance, whence the name.

Hedgehog \Hedge"hog`\, n.
   1. (Zo["o]l.) A small European insectivore ({Erinaceus
      Europ[ae]us}), and other allied species of Asia and
      Africa, having the hair on the upper part of its body
      mixed with prickles or spines. It is able to roll itself
      into a ball so as to present the spines outwardly in every
      direction. It is nocturnal in its habits, feeding chiefly
      upon insects.

   2. (Zo["o]l.) The Canadian porcupine.[U.S]

   3. (Bot.) A species of {Medicago} ({M. intertexta}), the pods
      of which are armed with short spines; -- popularly so
      called. --Loudon.

   4. A form of dredging machine. --Knight.

   {Hedgehog caterpillar} (Zo["o]l.), the hairy larv[ae] of
      several species of bombycid moths, as of the Isabella
      moth. It curls up like a hedgehog when disturbed. See
      {Woolly bear}, and {Isabella moth}.

   {Hedgehog fish} (Zo["o]l.), any spinose plectognath fish,
      esp. of the genus {Diodon}; the porcupine fish.

   {Hedgehog grass} (Bot.), a grass with spiny involucres,
      growing on sandy shores; burgrass ({Cenchrus
      tribuloides}).

   {Hedgehog rat} (Zo["o]l.), one of several West Indian
      rodents, allied to the porcupines, but with ratlike tails,
      and few quills, or only stiff bristles. The hedgehog rats
      belong to {Capromys}, {Plagiodon}, and allied genera.

   {Hedgehog shell} (Zo["o]l.), any spinose, marine, univalve
      shell of the genus {Murex}.

   {Hedgehog thistle} (Bot.), a plant of the Cactus family,
      globular in form, and covered with spines
      ({Echinocactus}).

   {Sea hedgehog}. See {Diodon}.

Source : WordNet®

hedgehog
     n 1: relatively large rodents with sharp erectile bristles
          mingled with the fur [syn: {porcupine}]
     2: small nocturnal Old World mammal covered with both hair and
        protective spines [syn: {Erinaceus europaeus}, {Erinaceus
        europeaeus}]