Prior restraint on speech is a legal term for the judicial prevention of a statement or expression from being made public either in speech or publication. In most instances, it applies to the media. In the US, this is precisely what the First Amendment was designed to prevent, and it is a form of censorship. In many US court rulings in which prior restraint is challenged, the First Amendment is upheld and the act is interpreted as censorship.
Generally speaking, prior restraint is the result of a government either learning about the intended publication of specific material or foreseeing the publication of such material and, in either case, preventing it. Though it has been questioned many times throughout history, the notion went largely unchallenged in the US until the landmark case of Near vs. Minnesota in 1931. With the advancement of technology and the ability to deliver information faster than ever, it's likely that this form of censorship will face new challenges in the judicial system, but an example that is rarely challenged involves embedded news correspondents during war. Reporters who travel with troops during war are prohibited from reporting certain information, including the specific location and intended military actions of the troop or unit they accompany.
Another example applies to high profile criminal proceedings. A judge can issue a gag order, which prohibits everyone involved in the trial and bearing witness to the trial from speaking about it publicly. The intention in such a situation is to ensure that the defendant’s right to a fair and unbiased trial is maintained. This practice also prevents the press from releasing information to the public about a criminal investigation prior to trial.
Prior restraint almost always applies to the First Amendment, and other factors are considered when it is challenged. Factors such as libel and slander are usually a separate legal issue, however. Many cases that challenge this form of censorship and the First Amendment are tried at state levels, but some reach the United States Supreme Court. Rights as defined under the First Amendment have been largely upheld throughout American history, and while prior restraint is not always challenged, it is most often defined as unconstitutional when it is.