observer

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One who observes.
• noun One who is sent to observe and report on events or proceedings without directly participating in them.
• noun A crew member on a military aircraft who makes observations.
• noun A member of an armed force who watches and reports from an observation post.

from The Century Dictionary.

• noun In experimental psychology, the general term for the subject of a psychological experiment: opposed to experimenter, the general term for the person who arranges the conditions of the experiment.
• noun One who observes or takes notice; a spectator or looker-on: as, a keen observer.
• noun One who is engaged in habitual or systematic observation, as for scientific purposes; especially, one who is trained to make certain special observations with accuracy and under proper precautions: as, an astronomical observer; a corps of observers.
• noun One who observes or keeps any law, custom, regulation, or rite; one who practises, performs, or fulfils anything: as, a careful observer of the proprieties; an observer of the sabbath.
• noun One who watches with a view to serve; an obsequious attendant or admirer; hence, a toady; a sycophant.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who observes, or pays attention to, anything; especially, one engaged in, or trained to habits of, close and exact observation.
• noun One who keeps any law, custom, regulation, rite, etc.; one who conforms to anything in practice.
• noun One who fulfills or performs.
• noun obsolete A sycophantic follower.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun One who makes observations, monitors or takes notice
• noun One who adheres or follows laws, guidelines, etc.
• noun A person sent as a representative, to a meeting or other function to monitor but not to participate
• noun military A crew member on an aircraft who makes observations of enemy positions or aircraft
• noun military A sentry etc. manning an observation post

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a person who becomes aware (of things or events) through the senses
• noun an expert who observes and comments on something