The Collaborative International Dictionary of English v.0.48:

Rider \Rid"er\ (r[imac]d"[~e]r), n.
   1. One who, or that which, rides.
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   2. Formerly, an agent who went out with samples of goods to
      obtain orders; a commercial traveler. [Eng.]
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   3. One who breaks or manages a horse. --Shak.
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   4. An addition or amendment to a manuscript or other
      document, which is attached on a separate piece of paper;
      in legislative practice, an additional clause annexed to a
      bill while in course of passage; something extra or
      burdensome that is imposed.
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            After the third reading, a foolish man stood up to
            propose a rider.                      --Macaulay.
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            This [question] was a rider which Mab found
            difficult to answer.                  --A. S. Hardy.
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   5. (Math.) A problem of more than usual difficulty added to
      another on an examination paper.
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   6. [D. rijder.] A Dutch gold coin having the figure of a man
      on horseback stamped upon it.
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            His moldy money ! half a dozen riders. --J.
                                                  Fletcher.
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   7. (Mining) Rock material in a vein of ore, dividing it.
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   8. (Shipbuilding) An interior rib occasionally fixed in a
      ship's hold, reaching from the keelson to the beams of the
      lower deck, to strengthen her frame. --Totten.
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   9. (Naut.) The second tier of casks in a vessel's hold.
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   10. A small forked weight which straddles the beam of a
       balance, along which it can be moved in the manner of the
       weight on a steelyard.
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   11. A robber. [Obs. or Prov. Eng.] --Drummond.
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   Rider's bone (Med.), a bony deposit in the muscles of the
      upper and inner part of the thigh, due to the pressure and
      irritation caused by the saddle in riding.
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WordNet (r) 3.0 (2006):

rider
    n 1: a traveler who actively rides an animal (as a horse or
         camel)
    2: a clause that is appended to a legislative bill
    3: a traveler who actively rides a vehicle (as a bicycle or
       motorcycle)
    4: a traveler riding in a vehicle (a boat or bus or car or plane
       or train etc) who is not operating it [syn: passenger,
       rider]

Moby Thesaurus II by Grady Ward, 1.0:

104 Moby Thesaurus words for "rider":
   addendum, affix, allonge, appendix, bidet, bill, broncobuster,
   buckaroo, caballero, calendar, carriage horse, cart horse,
   cavalier, cavalry horse, cavalryman, chevalier, circus rider,
   clause, coda, codicil, commentary, companion bills amendment,
   cowboy, cowgirl, cowpuncher, draft horse, dragnet clause,
   dray horse, driving horse, enacting clause, enclitic, envoi,
   epilogue, equestrian, equestrienne, escalator clause, fill horse,
   filler, gaucho, gigster, hack, hackney, hold-up bill,
   horse soldier, horseback rider, horsebacker, horseman, horsewoman,
   hunter, infix, interlineation, interpolation, jockey, joker,
   jument, knight, lead, leader, marginalia, motion, mount,
   mounted policeman, note, omnibus bill, pack horse, palfrey,
   plow horse, pole horse, polo pony, post-horse, postboy, postilion,
   postscript, prefix, privileged question, proclitic, proviso,
   puncher, question, remount, riding horse, road horse, roadster,
   roughrider, rouncy, saddle horse, saddler, saving clause, scholia,
   shaft horse, stalking-horse, steeplechaser, suffix, sumpter,
   sumpter horse, supplement, tail, thill horse, thiller, trick rider,
   vaquero, wheeler, wheelhorse, workhorse



Bouvier's Law Dictionary, Revised 6th Ed (1856):

RIDER, practice, legislation. A schedule or small piece of paper or
parchment added to some part of the record; as, when, on the reading of a
bill in the legislature, a new clause is added, this is tacked to the bill
on a separate piece of paper, and is called a rider.