noun

• (computing) A person who uses a computer or a computing network, especially a person who has received a user account.
• (Internet, informal) Clipping of username.
• (derogatory) An exploiter, an abuser (a person who exploits others, that is treats and regards people unfairly, selfishly or unethically).
• (law, dated) 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.
  "2012, R. (Stephen Malpass) v Durham County Council, [2012] EWHC 1934 (Admin) http://www.bailii.org/ew/cases/EWHC/Admin/2012/1934.html As to evidence of user... As to quality of user (i.e. was use by the public "as of right"), the inspector found that the grass over the whole of the application land has been regularly cut... ...which the inspector did not find sufficient of itself to render user permissive. Moreover, the defendant could not, the inspector advised, rely on communication to users that access to the land was regulated. Deferment to users of the organised pitches..."

noun

• (programming, slang) Someone who uses the R programming language.