What is "disassimilation"

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.

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

n. The decomposition of complex substances, within an organism, into simpler ones suitable only for excretion, with the release of energy; a normal nutritional process that is the reverse of assimilation.