An atrocity is a shocking and extremely wicked or cruel act or behavior, typically one involving physical violence or injury. An atrocity crime is such an act or behavior that is a violation of international criminal law falling under three historically defined in international law as crimes of genocide, war crimes, and crimes against humanity. Many legal scholars and international nongovernmental organizations (NGOs) working in the field regard ethnic cleansing as a fourth mass atrocity crime, despite it not yet being recognized as an independent crime under international law.

Crimes of aggression, also known as crimes against peace, are considered by some to be mass atrocity crimes. Along with atrocity crimes, they are included [AR] in the jurisdiction of the International Criminal Court. However, most legal scholars do not consider them to be mass atrocity crimes. While aggression is certainly a grave violation of international law and frequently the context in which mass atrocity crimes take place, the crime of aggression is distinguishable as an attack on the territory, sovereignty, or political independence of a state rather than on individuals.