Government action that prevents speech or publication before it occurs, rather than punishing it afterward. It is considered the most serious infringement of First Amendment rights and is subject to a strong presumption against its constitutionality.
From Latin 'prior' (former, earlier) and 'restrahere' (to hold back). The concept traces to English common law's rejection of licensing schemes for printing, with the constitutional principle developing from Blackstone's commentaries and crystallizing in 20th-century American jurisprudence.
Prior restraint is the constitutional equivalent of 'guilty until proven innocent' for speech—it's so dangerous to free expression that it's been called the most serious infringement of First Amendment rights! The Pentagon Papers case epitomized this principle when the Supreme Court refused to stop publication of classified Vietnam War documents, establishing that even national security rarely justifies prior restraint.
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