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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