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.

adjective

alert , attentive , aware

adjective

absent , absorbed , abstracted , airheaded , bemused , careless , daydreaming , distracted , distrait , dreaming , dreamy , engrossed , faraway , forgetful , goofing off , head in the clouds , heedless , inattentive , inconscient , lost , mooning , moony , oblivious , out to lunch , pipe dreaming , preoccupied , remote , removed , scatterbrained , space cadet , spacey , unaware of events , surroundings , unconscious , unheeding , unmindful , unobservant , unthinking , withdrawn , woolgathering