The Collaborative International Dictionary of English v.0.48:

Postage \Post"age\, n.
   The price established by law to be paid for the conveyance of
   a letter or other mailable matter by a public post.
   [1913 Webster]

   Postage stamp, a government stamp required to be put upon
      articles sent by mail in payment of the postage, esp. an
      adhesive stamp issued and sold for that purpose.
      [1913 Webster]

WordNet (r) 3.0 (2006):

postage stamp
    n 1: a small adhesive token stuck on a letter or package to
         indicate that that postal fees have been paid [syn:
         postage, postage stamp, stamp]