Source : Webster's Revised Unabridged Dictionary (1913)

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

   2. Loss of remembrance or recollection; a ceasing to
      remember; oblivion.

            A sweet forgetfulness of human care.  --Pope.

   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.

Source : WordNet®

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