Dictionary:

pastern

  (păs'tərn)

1. The part of a horse's foot between the fetlock and hoof.
2. An analogous part of the leg of a dog or other quadruped.

[Alteration of Middle English pastron, hobble, pastern, from Old French pasturon, diminutive of pasture, pasture, tether, alteration of *pastoire, from Latin pāstōria, feminine sing. of pāstōrius, of herdsmen, from pāstor, shepherd. See pastor.]

In ungulates the segment of the limb between the fetlock and hoof, supported by proximal and middle phalanges; in dogs the metacarpal region.

• equine p. dermatitis — see greasy heel.
• congenital p. flexure — a common neonatal deformity in all species. If it is the only deformity it commonly corrects itself spontaneously or with minimal interference. Occurs also as part of multiple deformities, e.g. Akabane virus disease.
• p. folliculitis — folliculitis of the posterior pastern in horses from which Staphylococcus hyicus has been isolated.
• p. joint — in ungulates, the articulation between the proximal and middle phalanges.

The noun has one meaning:

Meaning #1: the part between the fetlock and the hoof
  Synonym: fetter bone