observer

One who observes.

One who is sent to observe and report on events or proceedings without directly participating in them.

A crew member on a military aircraft who makes observations.

A member of an armed force who watches and reports from an observation post.

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.

One who observes or takes notice; a spectator or looker-on: as, a keen observer.

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.

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.

One who watches with a view to serve; an obsequious attendant or admirer; hence, a toady; a sycophant.