Wikipedia
Prior restraint (also referred to as prior censorship or pre-publication censorship) is censorship imposed, usually by a government, on expression before the expression actually takes place. An alternative to prior restraint is to allow the expression to take place and to take appropriate action afterward, if the expression is found to violate the law, regulations, or other rules.
Prior restraint prevents the censored material from being heard or distributed at all; other measures provide sanctions only after the offending material has been communicated, such as suits for slander or libel. In some countries (e.g., United States, Argentina) prior restraint by the government is forbidden, subject to certain exceptions, by a constitution.
Prior restraint can be effected in a number of ways. For example, the exhibition of works of art or a movie may require a license from a government authority (sometimes referred to a classification board or censorship board) before it can be published, and the failure or refusal to grant a license is a form of censorship as is the revoking of a license. It can also take the form of a legal injunction or government order prohibiting the publication of a specific document or subject. Sometimes, a government or other party becomes aware of a forthcoming publication on a particular subject and seeks to prevent it: to halt ongoing publication and prevent its resumption. These injunctions are considered prior restraint because potential future publications are stopped in advance.