Source : Webster's Revised Unabridged Dictionary (1913)

Rider \Rid"er\, n.
   1. One who, or that which, rides.

   2. Formerly, an agent who went out with samples of goods to
      obtain orders; a commercial traveler. [Eng.]

   3. One who breaks or manages a horse. --Shak.

   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.

            After the third reading, a foolish man stood up to
            propose a rider.                      --Macaulay.

            This [question] was a rider which Mab found
            difficult to answer.                  --A. S. Hardy.

   5. (Math.) A problem of more than usual difficulty added to
      another on an examination paper.

   6. [D. rijder.] A Dutch gold coin having the figure of a man
      on horseback stamped upon it.

            His moldy money ! half a dozen riders. --J.
                                                  Fletcher.

   7. (Mining) Rock material in a vein of ore, dividing it.

   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.

   9. (Naut.) The second tier of casks in a vessel's hold.

   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.

   11. A robber. [Obs. or Prov. Eng.] --Drummond.

   {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.

Source : WordNet®

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}]