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]

Moby Thesaurus II by Grady Ward, 1.0:

89 Moby Thesaurus words for "binder":
   Ace bandage, Band-Aid, acquitment, acquittal, acquittance,
   adhesive tape, amortization, amortizement, application, band,
   bandage, bandaging, binding, brace, cash, cash payment, cast,
   cataplasm, clearance, compress, cotton, court plaster, cravat,
   debt service, defrayal, defrayment, deposit, disbursal, discharge,
   doling out, down payment, dressing, dust jacket, earnest,
   earnest money, elastic bandage, envelope, envelopment, epithem,
   four-tailed bandage, gauze, gift wrapping, hire purchase,
   hire purchase plan, installment, installment plan,
   interest payment, jacket, lint, liquidation, monthly payments,
   never-never, paying, paying off, paying out, paying up, payment,
   payment in kind, payoff, plaster, plaster cast, pledget, poultice,
   prepayment, quarterly payments, quittance, regular payments,
   remittance, retirement, roller, roller bandage, rubber bandage,
   satisfaction, settlement, sinking-fund payment, sling, splint,
   sponge, spot cash, stupe, tampon, tape, tent, tourniquet,
   triangular bandage, weekly payments, wrap, wrapper, wrapping