Source : Webster's Revised Unabridged Dictionary (1913)

Lemming \Lem"ming\ (l[e^]m"m[i^]ng), n. [Nor. lemming, lemende;
   cf. Sw. lemel, Lapp. lummik.] (Zo["o]l.)
   Any one of several species of small arctic rodents of the
   genera {Myodes} and {Cuniculus}, resembling the meadow mice
   in form. They are found in both hemispheres.

   Note: The common Northern European lemming ({Myodes lemmus})
         is remarkable for making occasional devastating
         migrations in enormous numbers from the mountains into
         the lowlands.

Source : WordNet®

lemming
     n : any of various short-tailed furry-footed rodents of
         circumpolar distribution