preoccupied, inattentive, unattentive, absorbed, unmindful, absent, off, withdrawn, unheeding, heedless, unheedful, inadvertent; distracted, abstracted, day-dreaming, in a brown study, in the clouds, unaware, oblivious, in a trance, distrait(e), mooning, (far) away (somewhere), star-gazing, wool-gathering:
The absent-minded professor delivered his lecture to an empty lecture hall.
Advanced English Dictionary
+adjective tending to forget things, perhaps because you are not thinking about what is around you, but about sth else: Grandpa's becoming quite absent-minded. absent-mindedly adverb: She absent-mindedly twisted a strand of hair around her fingers. absent-mindedness noun [U]
Random quote: Discovery consists of seeing what everybody has seen and thinking what nobody else has thought.: Jonathan Swift