NOUN (3)

1. the process of decomposing organic matter (as in sewage) by bacteria or by chemical action or heat;

2. the organic process by which food is converted into substances that can be absorbed into the body;

3. learning and coming to understand ideas and information;
- Example: "his appetite for facts was better than his digestion"

The Collaborative International Dictionary of English v.0.48:

Digestion \Di*ges"tion\ (?; 106), n. [F. digestion, L.
   digestio.]
   1. The act or process of digesting; reduction to order;
      classification; thoughtful consideration.
      [1913 Webster]

   2. (Physiol.) The conversion of food, in the stomach and
      intestines, into soluble and diffusible products, capable
      of being absorbed by the blood.
      [1913 Webster]

   3. (Med.) Generation of pus; suppuration.
      [1913 Webster]

WordNet (r) 3.0 (2006):

digestion
    n 1: the process of decomposing organic matter (as in sewage) by
         bacteria or by chemical action or heat
    2: the organic process by which food is converted into
       substances that can be absorbed into the body
    3: learning and coming to understand ideas and information; "his
       appetite for facts was better than his digestion"

The Devil's Dictionary (1881-1906):

DIGESTION, n.  The conversion of victuals into virtues.  When the
process is imperfect, vices are evolved instead -- a circumstance from
which that wicked writer, Dr. Jeremiah Blenn, infers that the ladies
are the greater sufferers from dyspepsia.