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