The Collaborative International Dictionary of English v.0.48:

Absent-minded \Ab`sent-mind"ed\, a.
   Absent in mind; abstracted; preoccupied. --
   Ab`sent-mind"ed*ness, n. -- Ab`sent-mind"ed*ly, adv.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

79 Moby Thesaurus words for "absentmindedness":
   Lethe, Walter Mitty, absence of mind, absorption, abstractedness,
   abstraction, bemusement, brown study, carelessness,
   castle-building, daydream, daydreamer, daydreaming,
   decay of memory, depth of thought, disregard, disregardfulness,
   distraction, dream, dreaming, engrossment, fantasy, fantasying,
   fit of abstraction, flightiness, flippancy, forgetfulness,
   forgetting, forgiveness, frivolousness, giddiness,
   hazy recollection, heedlessness, inadvertence, inadvertency,
   inattention, inattentiveness, inconsideration, incuriosity,
   indifference, inobservance, levity, lightmindedness, mooning,
   moonraking, muse, musefulness, musing, muted ecstasy, negligence,
   nepenthe, nirvana, nonobservance, obliteration, oblivion,
   obliviousness, pipe dream, pipe-dreaming, preoccupation,
   regardlessness, reverie, shallowness, short memory, stargazing,
   study, superficiality, thoughtlessness, trance, unalertness,
   unawareness, unconsciousness, unheedfulness, unintentiveness,
   unmindfulness, unobservance, unwariness, unwatchfulness,
   waters of oblivion, woolgathering