Definition of "observer"

One who makes observations, monitors or takes notice  examples

One who adheres or follows laws, guidelines, etc.  examples

A person sent as a representative, to a meeting or other function to monitor but not to participate  examples

A country or other entity which has limited participation rights within an organization.  examples

(military) A crew member on an aircraft who makes observations of enemy positions or aircraft

(military) A sentry etc. manning an observation post