Prior restraint is when the media is prevented from publishing or is forced to clear information with the government before publishing (1).
It is distinguished from censorship in that the content (speech, art, etc.) is checked and restricted by the authorities in power before it is released, whereas censorship is the limitation of content which has already been published.
In the U.S., prior restraint is forbidden by the First Amendment of the Constitution, because it would mean that the government is hampering free speech.