The Collaborative International Dictionary of English v.0.48:

Coupon \Cou"pon\ (k??"p?n; F. k??`p?n"), n. [F., fr. couper to
   cut, cut off. See Coppice.]
   1. (Com.) A certificate of interest due, printed at the
      bottom of transferable bonds (state, railroad, etc.),
      given for a term of years, designed to be cut off and
      presented for payment when the interest is due; an
      interest warrant.
      [1913 Webster]

   2. A section of a ticket, showing the holder to be entitled
      to some specified accomodation or service, as to a passage
      over a designated line of travel, a particular seat in a
      theater, or the like.
      [1913 Webster]

   3. a ticket or form allowing the bearer to purchase a
      specific quantity of rationed goods; -- such coupons are
      typically issued by a governmental agency.
      [PJC]

   4. a printed form, often distributed as part of an
      advertisement, entitling the bearer to purchase a specific
      item of merchandise at a discount; or, a similar form
      which may be submitted to the issuer to obtain goods or
      information. Such coupons may be issued by the
      manufacturer (manufacturer's coupon) or by a retail
      establishment; in the latter case they may then be
      redeemed only at that particular retail store.
      [PJC]

WordNet (r) 3.0 (2006):

coupon
    n 1: a negotiable certificate that can be detached and redeemed
         as needed [syn: coupon, voucher]
    2: a test sample of some substance