Possible meanings:

1. In general, an observer is any system which receives information from an object.
2. An observer in an organization is a delegate sent to observe and report on the proceedings of an assembly or a meeting but not vote or otherwise participate.
3. An observer is an aircrew member of a (military) aircraft responsible for searching (usually visually) for intelligence, opposing forces and survivors of interest to the mission.
4. The Observer is a United Kingdom Sunday newspaper.
5. "The Observer" is the title of a track from the Flaming Lips' 1999 album The Soft Bulletin.
6. Observer pattern
7. An observer; something that does, has, or will observe.