observer

IPA: ʌbzˈɝvɝ

One who makes observations, monitors or takes notice
One who adheres or follows laws, guidelines, etc.
A person sent as a representative, to a meeting or other function to monitor but not to participate
A country or other entity which has limited participation rights within an organization.
(military) A crew member on an aircraft who makes observations of enemy positions or aircraft
(military) A sentry etc. manning an observation post

Examples of "observer" in Sentences

Becoming aware of oneself as observer is the death of observation.
Disinterested observer is right; I was talking about the Port Arthur gun laws.
As an observer from the US, it is a privilege to watch these women grow and stretch.
Typically we will pretend the observer is a person, though it could of course be a scientific apparatus as well.
Lets take this sentence as an example where an observer is looking at a pool of cars and notes that there is no red car in the pool.