A person who watches or notices something. The observer noted the changes in the weather patterns.
A person who attends a conference, meeting, or other gathering to listen and watch but not to participate. As an observer at the conference, she was not allowed to vote on any resolutions.
A person who is sent to a foreign country to monitor and report on the situation there, often in a diplomatic or military context. The United Nations sent an observer to the region to assess the humanitarian needs.