noun One who uses or makes use of something, a consumer or client or an express or implied licensee (free user) or a trespasser.
noun A person who uses drugs, especially illegal drugs.
noun A person who uses a computer or a computing network, especially a person who has received a user account.
noun Clipping of username.
noun An exploiter, an abuser (a person who exploits others, that is treats and regards people unfairly, selfishly or unethically).
noun In land law, meaning either 1. or 2. above or use. Usually in singular form to mean use wherever there is assiduous re-use of precedents and aloof textbooks verbatim. (clarification of this definition is needed)