(1):
(n.) One who fulfills or performs; as, an observer of his promises.
(2):
(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.
(3):
(n.) One who keeps any law, custom, regulation, rite, etc.; one who conforms to anything in practice.
(4):
(n.) A sycophantic follower.