Genocide is violence that targets individuals because of their membership of a group and aims at the destruction of a people. Raphael Lemkin, who coined the term, defined genocide as "the destruction of a nation or of an ethnic group" by means such as "the disintegration of [its] political and social institutions, of [its] culture, language, national feelings, religion, and [its] economic existence". During the struggle to ratify the Genocide Convention, powerful countries restricted Lemkin's definition to exclude their own actions from being classified as genocide, ultimately limiting it to any of five "acts committed with intent to destroy, in whole or in part, a national, ethnical, racial or religious group". While there are many scholarly definitions of genocide, almost all international bodies of law officially adjudicate the crime of genocide pursuant to the Genocide Convention.

Genocide has occurred throughout human history, even during prehistoric times, but it is particularly likely in situations of imperial expansion and power consolidation. It is associated with colonial empires and settler colonies, as well as with both world wars and repressive governments in the twentieth century. The colloquial understanding of genocide is heavily influenced by the Holocaust as its archetype and is conceived as innocent victims being targeted for their ethnic identity rather than for any political reason.

Genocide is widely considered to be the epitome of human evil and is often referred to as the "crime of crimes"; consequently, events are often denounced as genocide.

Polish-Jewish lawyer Raphael Lemkin coined the term genocide between 1941 and 1943. Lemkin's coinage combined the Greek word γένος (genos, "race, people") with the Latin suffix -caedo ("act of killing"). He submitted the manuscript for his book Axis Rule in Occupied Europe to the publisher in early 1942 and it was published in 1944 as the Holocaust was coming to light outside Europe. Lemkin's proposal was more ambitious than simply outlawing this type of mass slaughter. He also thought that the law against genocide could promote more tolerant and pluralistic societies. His response to Nazi criminality was sharply different from that of another international law scholar, Hersch Lauterpacht, who argued that it was essential to protect individuals from atrocities whether or not they were targeted as members of a group.

According to Lemkin, the central definition of genocide was "the destruction of a nation or of an ethnic group" in which its members were not targeted as individuals, but rather as members of the group. The objectives of genocide "would be the disintegration of the political and social institutions, of culture, language, national feelings, religion, and the economic existence of national groups". These were not separate crimes but different aspects of the same genocidal process. Lemkin's definition of nation was sufficiently broad to apply to nearly any type of human collectivity, even one based on a trivial characteristic. He saw genocide as an inherently colonial process, and in his later writings analyzed what he described as the colonial genocides occurring within European overseas territories as well as the Soviet and Nazi empires. Furthermore, his definition of genocidal acts, which was to replace the national pattern of the victim with that of the perpetrator, was much broader than the five types enumerated in the Genocide Convention. Lemkin considered genocide to have occurred since the beginning of human history and dated the efforts to criminalize it to the Spanish critics of colonial excesses Francisco de Vitoria and Bartolomé de Las Casas. The 1946 judgement against Arthur Greiser issued by a Polish court was the first legal verdict that mentioned the term, using Lemkin's original definition.

According to the legal instrument used to prosecute defeated German leaders at the International Military Tribunal at Nuremberg, atrocity crimes were only prosecutable by international justice if they were committed as part of an illegal war of aggression. The powers prosecuting the trial were unwilling to restrict a government's actions against its own citizens.

In order to criminalize peacetime genocide, Lemkin brought his proposal to criminalize genocide to the newly established United Nations in 1946. Opposition to the convention was greater than Lemkin expected due to states' concerns that it would lead their own policies—including treatment of indigenous peoples, European colonialism, racial segregation in the United States, and Soviet nationalities policy—to be labeled genocide. Before the convention was passed, powerful countries (both Western powers and the Soviet Union) secured changes in an attempt to make the convention unenforceable and applicable to their geopolitical rivals' actions but not their own. Few formerly colonized countries were represented and "most states had no interest in empowering their victims– past, present, and future".

The result severely diluted Lemkin's original concept; he privately considered it a failure. Lemkin's anti-colonial conception of genocide was transformed into one that favored colonial powers. Among the violence freed from the stigma of genocide was the destruction of political groups, which the Soviet Union is particularly blamed for blocking. Although Lemkin credited women's NGOs with securing the passage of the convention, the gendered violence of forced pregnancy, marriage, and divorce was left out. Additionally omitted was the forced migration of populations—which had been carried out by the Soviet Union and its allies, condoned by the Western powers, against millions of Germans from central and Eastern Europe.