Lynching is an extrajudicial killing by a group. It is most often used to characterize informal public executions by a mob in order to punish an alleged or convicted transgressor or to intimidate others. It can also be an extreme form of informal group social control, and it is often conducted with the display of a public spectacle (often in the form of a hanging) for maximum intimidation. Instances of lynchings and similar mob violence can be found in all societies.

In the United States, where the word lynching likely originated, the practice became associated with vigilante justice on the frontier and mob attacks on African Americans accused of crimes. The latter became frequent in the South during the period after the Reconstruction era, especially during the nadir of American race relations. Black people were the primary victims of lynching in the U.S. (about 72% of the total), which was often perpetrated to enforce white supremacy and intimidate ethnic minorities along with other acts of racial terrorism.

The origins of the word lynch are obscure, but it likely originated during the American Revolution. The verb comes from the phrase Lynch Law, a term for a punishment without trial. Two Americans during this era are generally credited for coining the phrase: Charles Lynch (1736–1796) and William Lynch (1742–1820), both of whom lived in Virginia in the 1780s. Charles Lynch is more likely to have coined the phrase, as he was known to have used the term in 1782, while William Lynch is not known to have used the term until much later. There is no evidence that death was imposed as a punishment by either of the two men. In 1782, Charles Lynch wrote that his assistant had administered Lynch's law to Tories "for Dealing with the negroes &c".

Charles Lynch was a Virginia Quaker, planter, and Patriot who headed a county court in Virginia which imprisoned Loyalists during the American Revolutionary War, occasionally imprisoning them for up to a year. Although he lacked proper jurisdiction for detaining these persons, he claimed this right by arguing wartime necessity. Lynch was concerned that he might face legal action from one or more of those whom he had imprisoned, notwithstanding that the Patriots had won the war. In 1780, he persuaded the Continental Congress to pass Lynch's Law to forgive extrajudicial wartime Loyalist imprisonment. It was in connection with this that the term Lynch law, meaning the assumption of extrajudicial authority, came into common parlance in the United States. Lynch was not accused of racist bias. He acquitted Black people accused of murder on three occasions. He was accused, however, of ethnic prejudice in his handling of Welsh miners.

William Lynch from Virginia claimed that the phrase was first used in a 1780 compact signed by him and his neighbors in Pittsylvania County.

A 17th-century legend of James Lynch fitz Stephen, who was Mayor of Galway in Ireland in 1493, says that when his son was convicted of murder, the mayor hanged him from his own house. The story was proposed by 1904 as the origin of the word "lynch". It is dismissed by etymologists, both because of the distance in time and place from the alleged event to the word's later emergence, and because the incident did not constitute a lynching in the modern sense.

The archaic verb linch, to beat severely with a pliable instrument, to chastise or to maltreat, has been proposed as the etymological source; but there is no evidence that the word has survived into modern times, so this claim is also considered implausible.

Since the 1970s, and especially since the 1990s, there has been a false etymology claiming that the word lynching comes from a fictitious William Lynch speech that was given by an especially brutal slaveholder to other slaveholders to explain how to control their slaves. Although a real person named William Lynch might have been the origin of the word lynching, the real life William Lynch definitely did not give this speech, and it is unknown whether the real William Lynch even owned slaves at all.