The Collaborative International Dictionary of English v.0.48:

Instep \In"step\, n. [Formerly also instop, instup.]
   [1913 Webster]
   1. The arched middle portion of the human foot next in front
      of the ankle joint.
      [1913 Webster]

   2. The part of a shoe, boot, or stocking covering the
      instep[1].
      [PJC]

   3. That part of the hind leg of the horse and allied animals,
      between the hock, or ham, and the pastern joint.
      [1913 Webster]

WordNet (r) 3.0 (2006):

instep
    n 1: the arch of the foot
    2: the part of a shoe or stocking that covers the arch of the
       foot

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "instep":
   arch, clubfoot, digit, dog, extremity, fetlock, foot, forefoot,
   forepaw, harefoot, heel, hoof, pad, pastern, patte, paw,
   pedal extremity, pedes, pes, pied, pug, sole, splayfoot, toe,
   tootsy, trotter, ungula