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]