observer

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.”