Observer

From {1} + -er.

Full definition of observer

(plural observers)

One who makes observations, monitors or takes noticeMost impartial observers agreed that Sampras had not served well.
One who adheres or follows laws, guidelines, etc.I shall be an observer of the local customs.
A person sent as a representative, to a meeting or other function to monitor but not to participateThe UN sent many observers to the country's first elections.
(military) A crew member on an aircraft who makes observations of enemy positions or aircraftThe only crew-member to survive the crash was the Canadian observer.
(military) A sentry etc. manning an observation postWe waited till dusk when the observers' vision was poorest.