A heckler's veto occurs when an acting party's right to freedom of speech is curtailed or restricted by the government in order to prevent a reacting party's behavior. The common example is that of demonstrators (reacting party) causing a speech (given by the acting party) to be terminated in order to preserve the peace.
The term was coined by University of Chicago professor of law Harry Kalven.
In the United States, case law regarding the heckler's veto is mixed. Most findings say that the acting party's actions cannot be pre-emptively stopped due to fear of heckling by the reacting party, but in the immediate face of violence, authorities can ask the acting party to cease their action in order to satisfy the hecklers.
The best known case involving the heckler's veto is probably Feiner v. New York, handed down by the Supreme Court in 1951. Chief Justice Fred M. Vinson, writing for the majority, held that police officers acted within their power in arresting a speaker if the arrest was "motivated solely by a proper concern for the preservation of order and protection of the general welfare." 340 U.S. 315.
It was rejected in Hill v. Colorado, where the Supreme Court rejected the "Heckler's Veto," finding "governmental grants of power to private actors" to be "constitutionally problematic" in cases where "the regulations allowed a single, private actor to unilaterally silence a speaker"