Observer

n. one who observes; spectator; onlooker; keeper of religious law

In information theory, an observer is any system which receives information from an object.Observer can have the following meanings:A person that is observing, its role in observational sciences and physical reference frames: see observationA delegate in an organization that is sent to observe and report on the proceedings of an assembly or a meeting but that can not vote or otherwise participate. An aircrew member of a (military) aircraft responsible for searching (usually visually) for intelligence, opposing forces and survivors of interest to the mission A state observer used in control applications. In most control systems not all state variables of the plant can be measured. If it is an observable plant the state can be estimated with the help of a state observer. The estimated state is often used for full state feedback.A UK Sunday newspaper: see The ObserverThe Texas Observer, an American political magazineA Sri Lankan Sunday and evening newspaper: See List of newspapers in Sri LankaA design pattern used in computer programming: see Observer pattern"The Observer", a track from the Flaming Lips' 1999 album The Soft BulletinAn observer, an airborne reconnaissance robot used by the Protoss in the StarCraft series.One of many omnipotent aliens in Mystery Science Theater 3000, see Brain Guy
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Noun
1. a person who becomes aware (of things or events) through the senses
(synonym) perceiver, beholder
(hypernym) person, individual, someone, somebody, mortal, human, soul
(hyponym) eyeglass wearer
(derivation) watch, observe, follow, watch over, keep an eye on
2. an expert who observes and comments on something
(synonym) commentator
(hypernym) expert
(hyponym) annotator
(derivation) note, observe, mention, remark

v. observe, watch; examen; notice, comment, remark; adhere, keep, respect, obey

(n.)
One who observes, or pays attention to, anything; especially, one engaged in, or trained to habits of, close and exact observation; as, an astronomical observer.
  

(n.)
One who keeps any law, custom, regulation, rite, etc.; one who conforms to anything in practice.
  

(n.)
One who fulfills or performs; as, an observer of his promises.
  

(n.)
A sycophantic follower.