Source : Webster's Revised Unabridged Dictionary (1913)

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

   2. That part of the hind leg of the horse and allied animals,
      between the hock, or ham, and the pastern joint.

Source : WordNet®

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