The Collaborative International Dictionary of English v.0.48:

Postal \Post"al\, a. [Cf. F. postal.]
   Belonging to the post office or mail service; as, postal
   arrangements; postal authorities.
   [1913 Webster]

   Postal card, or Post card, a card used for transmission
      of messages through the mails, at a lower rate of postage
      than a sealed letter; also called postcard. Such cards
      are sold by the government with postage already paid, or
      by private vendors without a postage stamp. The message is
      written on one side of the card, and the address on the
      other.

   Postal money order. See Money order, under Money.

   Postal note, an order payable to bearer, for a sum of money
      (in the United States less than five dollars under
      existing law), issued from one post office and payable at
      another specified office.

   Postal Union, a union for postal purposes entered into by
      the most important powers, or governments, which have
      agreed to transport mail matter through their several
      territories at a stipulated rate.
      [1913 Webster]

WordNet (r) 3.0 (2006):

post card
    n 1: a card for sending messages by post without an envelope
         [syn: postcard, post card, postal card, mailing-
         card]