someone or something that observes.

a delegate to an assembly or gathering, who is sent to observe and report but not to take part officially in its activities.

U.S. Air Force .

a member of an aircrew, other than the pilot, holding an aeronautical rating.

a person who maintains observation in an aircraft during flight.

Also called air observer , aircraft observer. U.S. Army . a person who serves in an aircraft as a reconnoiterer and directs artillery fire.

noun

beholder , bystander , eyewitness , gaper , gazer , looker , looker-on , onlooker , viewer , watcher , witness , spectator , eyer , spy