Genocide is "the deliberate and systematic destruction of, in whole or in part, of an ethnic, racial, religious, or national group", though what constitutes enough of a "part" to qualify as genocide has been subject to much debate by legal scholars. While a precise definition varies among genocide scholars, a legal definition is found in the 1948 United Nations Convention on the Prevention and Punishment of the Crime of Genocide (CPPCG). Article 2 of this convention defines genocide as "any of the following acts committed with intent to destroy, in whole or in part, a national, ethnical, racial or religious group, as such: killing members of the group; causing serious bodily or mental harm to members of the group; deliberately inflicting on the group conditions of life, calculated to bring about its physical destruction in whole or in part; imposing measures intended to prevent births within the group; [and] forcibly transferring children of the group to another group."

Raphael Lemkin, in his work Axis Rule in Occupied Europe (1944), coined the term "genocide" by combining Greek genos (γένος; race, people) and Latin cīdere (to kill).

Lemkin defined genocide as follows: "Generally speaking, genocide does not necessarily mean the immediate destruction of a nation, except when accomplished by mass killings of all members of a nation. It is intended rather to signify a coordinated plan of different actions aiming at the destruction of essential foundations of the life of national groups, with the aim of annihilating the groups themselves. The objectives of such a plan would be the disintegration of the political and social institutions, of culture, language, national feelings, religion, and the economic existence of national groups, and the destruction of the personal security, liberty, health, dignity, and even the lives of the individuals belonging to such groups." The preamble to the CPPCG states that instances of genocide have taken place throughout history, but it was not until Raphael Lemkin coined the term and the prosecution of perpetrators of the Holocaust at the Nuremberg trials that the United Nations agreed to the CPPCG which defined the crime of genocide under international law.