Absent-minded(#), a.
Absent in mind; abstracted; preoccupied. -- Ab`sent-mind"ed*ness, n. -- Ab`sent-mind"ed*ly, adv.
Shades of meaning
Fernald has no entry of its own for absent-minded; he lists it under abstracted.
Where he writes about it:
As regards mental action, absorbed, abstracted, and preoccupied refer to the cause, absent or absent-minded to the effect. — from abstracted
The absent-minded man is oblivious of ordinary matters, because his thoughts are elsewhere. — from abstracted
One who is preoccupied is intensely busy in thought; one may be absent-minded either through intense concentration or simply through inattention, with fitful and aimless wandering of thought. — from abstracted