The Collaborative International Dictionary of English v.0.48:

Forgetfulness \For*get"ful*ness\, n.
   1. The quality of being forgetful; prononess to let slip from
      the mind.
      [1913 Webster]

   2. Loss of remembrance or recollection; a ceasing to
      remember; oblivion.
      [1913 Webster]

            A sweet forgetfulness of human care.  --Pope.
      [1913 Webster]

   3. Failure to bear in mind; careless omission; inattention;
      as, forgetfulness of duty.

   Syn: Forgetfulnes, Oblivion.

   Usage: Forgetfulness is Anglo-Saxon, and oblivion is Latin.
          The former commonly has reference to persons, and
          marks a state of mind; the latter commonly has
          reference to things, and indicates a condition into
          which they are sunk. We blame a man for his
          forgetfulness; we speak of some old custom as buried
          in oblivion. But this discrimination is not strictly
          adhered to.
          [1913 Webster]

WordNet (r) 3.0 (2006):

forgetfulness
    n 1: tendency to forget
    2: unawareness caused by neglectful or heedless failure to
       remember; "his forgetfulness increased as he grew older"

The Devil's Dictionary (1881-1906):

FORGETFULNESS, n.  A gift of God bestowed upon doctors in compensation
for their destitution of conscience.