Definitions
1.NounOne who uses or makes use of something, a consumer or client or an express or implied licensee (free user) or a trespasser.
2.NounA person who uses drugs, especially illegal drugs.
3.NounA person who uses a computer or a computing network, especially a person who has received a user account.
4.NounClipping of username.
5.NounAn exploiter, an abuser (a person who exploits others, that is treats and regards people unfairly, selfishly or unethically).
6.NounIn 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.