A legal restriction against the use of something or against certain conduct. For example, in the 1920s, both the USA and Canada enacted liquor prohibitions, outlawing the manufacture or use of alcoholic beverages (see poster, pictured).
Prohibition also has another meaning, within administrative law. As certiorari seeks to have a decision overturned for want of jurisdiction (ultra vires), prohibition seeks to prevent a prospective decision; to stop a lower court or tribunal "from doing something in excess of its jurisdiction".1
1. Wade, William, Administrative Law, 2000, Oxford University Press, page 592.