The Collaborative International Dictionary of English v.0.48:

Lemming \Lem"ming\ (l[e^]m"m[i^]ng), n. [Nor. lemming, lemende;
   cf. Sw. lemel, Lapp. lummik.] (Zool.)
   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.
   [1913 Webster]

   Note: The common Northern European lemming (Myodes lemmus)
         is remarkable for making occasional devastating
         migrations in enormous numbers from the mountains into
         the lowlands.
         [1913 Webster]