The Collaborative International Dictionary of English v.0.48:

Conservation \Con`ser*va"tion\, n. [L. conservatio: cf. F.
   conservation.]
   The act of preserving, guarding, or protecting; the keeping
   (of a thing) in a safe or entire state; preservation.
   [1913 Webster]

         A step necessary for the conservation of Protestantism.
                                                  --Hallam.
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         A state without the means of some change is without the
         means of its conservation.               --Burke.
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   Conservation of areas (Astron.), the principle that the
      radius vector drawn from a planet to the sun sweeps over
      equal areas in equal times.

   Conservation of energy, or Conservation of force (Mech.),
      the principle that the total energy of any material system
      is a quantity which can neither be increased nor
      diminished by any action between the parts of the system,
      though it may be transformed into any of the forms of
      which energy is susceptible. --Clerk Maxwell.
      [1913 Webster]

WordNet (r) 3.0 (2006):

conservation of energy
    n 1: the fundamental principle of physics that the total energy
         of an isolated system is constant despite internal changes
         [syn: conservation of energy, law of conservation of
         energy, first law of thermodynamics]