Heckler's veto

A Heckler's veto refers to, in the legal context, the suppression of free speech by the government or government actors due to a potential threat from those listening to the speech. It may be considered a form of incitement, but differs from fighting words because, in the case of fighting words, the speaker is attempting to cause violence, but under a heckler's veto the audience (or "heckler") may react violently even if that is not the purpose of the speech. It can also refer to, more generally, the curtailing of speech by force or threat of force.

Legally
Rulings involving the concept have been mixed, but over time have gone to favor a narrower range in which it is considered legal to arrest someone for potential violence resulting from their speech. While police officers can't arrest individuals based on the content of their speech, they can arrest them if there is a potential for imminent lawless action, even if the person is not purposefully trying to cause violence, as happened in the case of Feiner v. New York. However, it is not legal to prevent the speech in the first place because of fear of violence, or to charge money for police protection as a condition for allowing the speech.

Though various rulings have sided with the government defendants that they were justified in curtailing speech because of a legitimate fear of violence, they have also stressed that it would be more ideal to arrest those threatening violence. In what might be called the "no duh" principle, police have been reluctant to go up against angry mobs and often instead just arrest the speaker. This will probably keep happening until a court finally realizes that they've made it pseudo-legal for private citizens to suppress free speech if enough of them act angry and call an officer's bluff.

Other uses
Heckler's veto outside of legal circles can also be used to describe the occurrence of private citizens inhibiting speech they do not agree with. Situations that fall into this category range from small acts like an individual yelling loudly over a presentation so people have trouble hearing it to physical violence like a group of protesters attacking somebody giving a speech. While violent means are clearly illegal, non-violent disruptions are a bit trickier to classify. Booing an individual when they enter the stage is probably all right, as are wearing t-shirts with contradictory messages or holding your thumbs down at the presenter. Attempting to completely drown out a presenter at a public venue may or may not get you in trouble, especially with competing free speech, but you probably couldn't get away with it in a school environment (where the administrators have significantly more power to prevent disrupting the education of others).

It can also refer to a situations where individuals threaten others with violence ahead of time in the hopes of cancelling an event. This might be considered to be something like a bomb threat, but rather than to push a test back a few days you are trying to prevent something from happening at all. This is the social equivalent of the legal action of prior restraint, but instead of the government cancelling something because people might get violent, somebody willingly self-censors for their safety. This recently happened when Anita Sarkeesian cancelled a Utah State University lecture after she received death threats and the University could not legally guarantee her there would be no guns at the event due to concealed carry laws in the state.