observer

Etymology

From observe + -er.

noun

1. One who makes observations, monitors or takes notice
   Most impartial observers agreed that Sampras had not served well.
2. One who adheres or follows laws, guidelines, etc.
   I shall be an observer of the local customs.
3. A person sent as a representative, to a meeting or other function to monitor but not to participate
   The UN sent many observers to the country's first elections.
4. A country or other entity which has limited participation rights within an organization.
   The Vatican and Palestine have observer status at the UN.
5. (military) A crew member on an aircraft who makes observations of enemy positions or aircraft
   The only crew-member to survive the crash was the Canadian observer.
6. (military) A sentry etc. manning an observation post
   We waited till dusk when the observers' vision was poorest.