n. an attempt to prevent publication or broadcast of any statement, which is an unconstitutional restraint on free speech and free press (even in the guise of an anti-nuisance ordinance). Stemming from the First Amendment to the Constitution, the ban on prior restraint allows publication of libel, slander, obvious untruths, anti-government diatribes, racial and religious epithets, and almost any material, except if public security or public safety is endangered (false claim of poison in the reservoir or exhortation to commit a crime like a lynching) and some forms of pornography. The theory, articulated by the U.S. Supreme Court in Near v. Minnesota (1931) is that free speech and free press protections have priority, and lawsuits for libel and slander and prosecutions for criminal advocacy will curb the effect of defamation and untruths. Most other nations permit prior restraint by court order or police action when the material appears to be defamatory (hurtful lies), salacious (nasty), or "improper, mischievous, or illegal" (in the words of Sir William Blackstone).

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 Bayesian statistical inference, a prior probability distribution, often simply called the prior, of an uncertain quantity is the probability distribution that would express one's beliefs about this quantity before some evidence is taken into account. For example, the prior could be the probability distribution representing the relative proportions of voters who will vote for a particular politician in a future election.