The Collaborative International Dictionary of English v.0.48:

packet \pack"et\ (p[a^]k"[e^]t), n. [F. paquet, dim. fr. LL.
   paccus, from the same source as E. pack. See Pack.]
   [1913 Webster]
   1. A small pack or package; a little bundle or parcel; as, a
      packet of letters. --Shak.
      [1913 Webster]

   2. Originally, a vessel employed by government to convey
      dispatches or mails; hence, a vessel employed in conveying
      dispatches, mails, passengers, and goods, and having fixed
      days of sailing; a mail boat.
      [1913 Webster]

   Packet boat, Packet ship, or Packet vessel. See
      Packet, n., 2.

   Packet day, the day for mailing letters to go by packet; or
      the sailing day.

   Packet note or Packet post. See under Paper.
      [1913 Webster]

WordNet (r) 3.0 (2006):

packet boat
    n 1: a boat for carrying mail [syn: mailboat, mail boat,
         packet, packet boat]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "packet boat":
   carrier, carrier pigeon, homer, homing pigeon, mail car,
   mail coach, mail packet, mail train, mail truck, mailer, mailplane,
   pigeon post, post boat, post car, post coach, post-horse,
   post-office car, poster, railway mail car