so lost in thought that one does not realize what one is doing, what is happening, etc.; preoccupied to the extent of being unaware of one's immediate surroundings.
Origin: Related forms
withdrawn, musing, daydreaming, dreamy, forgetful, distracted. Absent-minded, abstracted, oblivious all mean inattentive to immediate surroundings. Absent-minded suggests an unintentional wandering of the mind from the present: an absent-minded committee member. Abstracted implies that the mind has been drawn away from the immediate present by reflection upon some engrossing subject: an abstracted air. Oblivious implies absorption in some thought that causes one to be completely forgetful of or unaware of one's surroundings: oblivious of danger.
attentive, alert, heedful, observant.