The descriptor "Open" is used in a number of sports, and especially in golf, to describe a tournament that is “open” (in theory to allcomers) rather than being closed (i.e. one that is restricted to a particular group). Thus an Amateur tournament (e.g. the U.S. Amateur) is not open because it is, by definition, closed to professional golfers. In golf the first precise usage of the adjective “Open” was in 1861 when the Prestwick Golf Club opened its Medal competition (launched the previous year for professionals only) to amateurs as well and ever since The Open Championship, as it was called, has been open to all. Any golfer can, in theory, play in The Open either by invitation or by various forms of qualification. In practice the event is a professional tournament in which a small number of the world’s leading amateurs also play (by invitation or qualification).
Over time the descriptor "Open" has been extended to include the premier national tournaments of many countries (starting with the U.S. Open in 1895) and also (mainly in the United States) to some other tournaments (e.g. the Buick Open).