Prior restraint (also referred to as prior censorship or pre-publication censorship) is censorship imposed, usually by a government or institution, on expression, that prohibits particular instances of expression. It is in contrast to censorship which establishes general subject matter restrictions and reviews a particular instance of expression only after the expression has taken place. In some countries (e.g., United States, Argentina) prior restraint by the government is forbidden, subject to exceptions (such as classifying certain matters of national security), by a constitution.