The Collaborative International Dictionary of English v.0.48:

Reservation \Res`er*va"tion\ (r?z`?r-v?"sh?n), n. [Cf. F.
   r['e]servation, LL. reservatio. See Reserve.]
   1. The act of reserving, or keeping back; concealment, or
      withholding from disclosure; reserve. --A. Smith.
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            With reservation of an hundred knights. --Shak.
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            Make some reservation of your wrongs. --Shak.
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   2. Something withheld, either not expressed or disclosed, or
      not given up or brought forward. --Dryden.
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   3. A tract of the public land reserved for some special use,
      as for schools, for the use of Indians, etc. [U.S.]
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   4. The state of being reserved, or kept in store. --Shak.
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   5. (Law)
      (a) A clause in an instrument by which some new thing is
          reserved out of the thing granted, and not in esse
          before.
      (b) A proviso. --Kent.
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   Note: This term is often used in the same sense with
         exception, the technical distinction being disregarded.
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   6. (Eccl.)
      (a) The portion of the sacramental elements reserved for
          purposes of devotion and for the communion of the
          absent and sick.
      (b) A term of canon law, which signifies that the pope
          reserves to himself appointment to certain benefices.
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   7. an agreement to have some space, service or other
      acommodation, as at a hotel, a restaurant, or on a public
      transport system, held for one's future use; also, the
      record or receipt for such an agreement, or the
      contractual obligation to retain that accommodation; as, a
      hotel reservation; a reservation on a flight to Dallas; to
      book a reservation at the Ritz.
      [PJC]

   Mental reservation, the withholding, or failing to
      disclose, something that affects a statement, promise,
      etc., and which, if disclosed, would materially change its
      import.
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WordNet (r) 3.0 (2006):

mental reservation
    n 1: an unstated doubt that prevents you from accepting
         something wholeheartedly [syn: mental reservation,
         reservation, arriere pensee]