The Collaborative International Dictionary of English v.0.48:

Binder \Bind"er\, n.
   1. One who binds; as, a binder of sheaves; one whose trade is
      to bind; as, a binder of books.
      [1913 Webster]

   2. Anything that binds, as a fillet, cord, rope, or band; a
      bandage; -- esp. the principal piece of timber intended to
      bind together any building.
      [1913 Webster]

   3. a pair of stiff oblong covers, sometimes detachable,
      designed for insertion of paper pages to create a
      book-like document, such as in a loose-leaf binder.
      [PJC]

WordNet (r) 3.0 (2006):

binder
    n 1: a machine that cuts grain and binds it in sheaves [syn:
         binder, reaper binder]
    2: something used to bind separate particles together or
       facilitate adhesion to a surface
    3: holds loose papers or magazines [syn: binder, ring-
       binder]
    4: something used to tie or bind [syn: binder, ligature]