Source : Webster's Revised Unabridged Dictionary (1913)

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.

            With reservation of an hundred knights. --Shak.

            Make some reservation of your wrongs. --Shak.

   2. Something withheld, either not expressed or disclosed, or
      not given up or brought forward. --Dryden.

   3. A tract of the public land reserved for some special use,
      as for schools, for the use of Indians, etc. [U.S.]

   4. The state of being reserved, or kept in store. --Shak.

   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.

   Note: This term is often used in the same sense with
         exception, the technical distinction being disregarded.

   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.

   {Mental reservation}, the withholding, or failing to
      disclose, something that affects a statement, promise,
      etc., and which, if disclosed, would materially change its
      import.

Source : WordNet®

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