The Collaborative International Dictionary of English v.0.48:

Pastern \Pas"tern\, n. [Of. pasturon, F. p[^a]turon, fr. OF.
   pasture a tether, for beasts while pasturing; prop., a
   pasturing. See Pasture.]
   1. The part of the foot of the horse, and allied animals,
      between the fetlock and the coffin joint. See Illust. of
      Horse.
      [1913 Webster]

   Note: The upper bone, or phalanx, of the foot is called the
         great pastern bone; the second, the small pastern
         bone; and the third, in the hoof, the coffin bone.
         [1913 Webster]

   Pastern joint, the joint in the hoof of the horse, and
      allied animals, between the great and small pastern bones.
      [1913 Webster]

   2. A shackle for horses while pasturing. --Knight.
      [1913 Webster]

   3. A patten. [Obs.] --Dryden.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pastern
    n 1: the part between the fetlock and the hoof [syn: pastern,
         fetter bone]