The Collaborative International Dictionary of English v.0.48:

Poster \Post"er\, n.
   1. One who posts, or travels expeditiously; a courier.
      "Posters of the sea and land." --Shak.
      [1913 Webster]

   2. A post horse. "Posters at full gallop." --C. Lever.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Poster \Post"er\, n.
   1. A large bill or placard intended to be posted in public
      places.
      [1913 Webster]

   2. One who posts bills; a billposter.
      [1913 Webster]

WordNet (r) 3.0 (2006):

poster
    n 1: a sign posted in a public place as an advertisement; "a
         poster advertised the coming attractions" [syn: poster,
         posting, placard, notice, bill, card]
    2: someone who pastes up bills or placards on walls or
       billboards [syn: bill poster, poster, bill sticker]
    3: a horse kept at an inn or post house for use by mail carriers
       or for rent to travelers [syn: post horse, post-horse,
       poster]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "poster":
   advertisement, affiche, announcement, banner, bill, billboard,
   broadside, carrier, carrier pigeon, circular, flier, handbill,
   homer, homing pigeon, mail car, mail coach, mail packet,
   mail train, mail truck, mailer, mailplane, notice, packet boat,
   pigeon post, placard, post boat, post car, post coach, post-horse,
   post-office car, railway mail car, sign, signboard