observer

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. One that observes: an observer of local customs; observers of religious holidays.
2. n. A delegate sent to observe and report on the proceedings of an assembly or a meeting but not vote or otherwise participate.
3. n. A crew member on a military aircraft who makes observations.
4. n. A member of an armed force who watches and reports from an observation post.

Century Dictionary and Cyclopedia

1. n. One who observes or takes notice; a spectator or looker-on: as, a keen observer.
2. n. 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.
3. n. 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.
4. n. One who watches with a view to serve; an obsequious attendant or admirer; hence, a toady; a sycophant.
5. n. 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.

Wiktionary

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

GNU Webster's 1913

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

WordNet 3.0

1. n. a person who becomes aware (of things or events) through the senses
2. n. an expert who observes and comments on something