Heckler’s veto is a form of curtailment of one's freedom of expression. It occurs when the government restricts the right to speak or to express ones opinion or reaction. It is commonly resorted to to avoid expression of negativity. Wikipedia defines heckler's veto as:
In the free speech context, a heckler's veto is either of two situations in which a person who disagrees with a speaker's message is able to unilaterally trigger events that result in the speaker being silenced.
In the strict legal sense, a heckler's veto occurs when the speaker's right is curtailed or restricted by the government in order to prevent a reacting party's behavior. The common example is the termination of a speech or demonstration in the interest of maintaining the public peace based on the anticipated negative reaction of someone opposed to that speech or demonstration. The term was coined by University of Chicago professor of law Harry Kalven.
In common parlance, the term is used to describe situations where hecklers or demonstrators silence a speaker without intervention of the law.