The Collaborative International Dictionary of English v.0.48:

groundhog \ground"hog`\, ground hog \ground" hog`\
   (ground"h[o^]g`), n.
   A reddish brown North American burrowing marmot (Marmota
   monax), also called the woodchuck. It hibernates in the
   winter.

   Syn: woodchuck, Marmota monax.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

marmot \mar"mot\ (m[aum]r"m[o^]t; 277), n. [It. marmotta,
   marmotto, prob. fr. L. mus montanus, or mus montis, lit.,
   mountain mouse or rat. See Mountain, and Mouse.]
   1. (Zool.) Any rodent of the genus Marmota (formerly
      Arctomys) of the subfamily Sciurinae. The common
      European marmot (Marmota marmotta) is about the size of
      a rabbit, and inhabits the higher regions of the Alps and
      Pyrenees. The bobac is another European species. The
      common American species (Marmota monax) is the
      woodchuck (also called groundhog), but the name marmot
      is usually used only for the western variety.
      [1913 Webster +PJC]

   2. Any one of several species of ground squirrels or gophers
      of the genus Spermophilus; also, the prairie dog.
      [1913 Webster]

   Marmot squirrel (Zool.), a ground squirrel or spermophile.


   Prairie marmot. See Prairie dog.
      [1913 Webster]

WordNet (r) 3.0 (2006):

groundhog
    n 1: reddish brown North American marmot [syn: groundhog,
         woodchuck, Marmota monax]

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "groundhog":
   Cape polecat, ape, bar, bear, cavy, chimp, chimpanzee, coon,
   digger, dredge, dredger, driller, excavator, ferret, foumart,
   glutton, guinea pig, hedgehog, miner, monk, monkey, mousehound,
   navvy, opossum, polecat, porcupine, possum, prairie dog, quill pig,
   raccoon, sandhog, sapper, skunk, steam shovel, tunneler, weasel,
   whistle-pig, wolverine, woodchuck, zoril