catabolism
    n 1: breakdown in living organisms of more complex substances
         into simpler ones together with release of energy [syn:
         catabolism, katabolism, dissimilation, destructive
         metabolism] [ant: anabolism, constructive metabolism]

Metabolism \Me*tab"o*lism\, n. (Physiol.)
   1. The act or process, by which living tissues or cells take
      up and convert into their own proper substance the
      nutritive material brought to them by the blood, or by
      which they transform their cell protoplasm into simpler
      substances, which are fitted either for excretion or for
      some special purpose, as in the manufacture of the
      digestive enzymes. Hence, metabolism may be either
      constructive (anabolism), or destructive (catabolism).
      [1913 Webster]

   2. (Biol.) The series of chemical changes which take place in
      an organism, by means of which food is manufactured and
      utilized and waste materials are eliminated.
      [Webster 1913 Suppl.]

Disassimilation \Dis`as*sim`i*la"tion\, n. (Physics)
   The decomposition of complex substances, within the organism,
   into simpler ones suitable only for excretion, with evolution
   of energy, -- a normal nutritional process the reverse of
   assimilation; downward metabolism; -- now more commonly
   called catabolism.
   [1913 Webster +PJC]

         The breaking down of already existing chemical
         compounds into simpler ones, sometimes called
         disassimilation.                         --Martin.
   [1913 Webster]

catabolism \catabolism\ n.
   the breakdown of more complex substances into simpler ones
   with release of energy, in living organisms; destructive or
   downward metabolism; -- a form of metabolism, opposed to
   anabolism. See also Disassimilation. [Formerly spelled
   katabolism.]

   Syn: katabolism, dissimilation, destructive metabolism.
        [WordNet 1.5]