Short answers, pulled from the story.
Raphael Lemkin, a Polish-Jewish lawyer, coined the term genocide between 1941 and 1943. He combined the Greek word genos, meaning race or people, with the Latin suffix -caedo, the act of killing. His interest in the subject was first sparked as a law student by the Armenian genocide.
The Genocide Convention defines genocide as any of five acts committed with intent to destroy, in whole or in part, a national, ethnical, racial or religious group, as such. The five acts are killing group members, causing serious bodily or mental harm, inflicting conditions of life calculated to destroy the group, imposing measures to prevent births, and forcibly transferring children to another group.
The United Nations General Assembly adopted the Genocide Convention on the 9th of December 1948. It came into effect on the 12th of January 1951, after 20 countries ratified it without reservations. It does not allow retroactive prosecution of events that took place before 1951.
Most perpetrators of genocide are psychologically normal because genocide is typically a state crime carried out through ordinary agents such as soldiers, bureaucracies, and paramilitaries. The foot soldiers are not demographically or psychologically aberrant, and they are rarely true believers, driven instead by obedience, conformity, and the diffusion of responsibility.
Khieu Samphan was the first former head of state convicted of genocide, in 2018, for the Cambodian genocide. The International Criminal Court, established in 2002, and earlier ad hoc tribunals also try individuals for genocide and other international crimes.
Cultural genocide, also called ethnocide, refers to actions that target a group's language, religion, cultural heritage, leaders, and traditional way of life rather than its physical existence. It was removed from the Genocide Convention despite Lemkin's argument that cultural and physical genocide were two mechanisms aiming at the same goal, and most genocide scholars believe it should be included when committed with intent to destroy the group.
Intervention against genocide has often failed because most countries prioritize business, trade, and diplomatic relationships, allowing powerful actors to use violence against vulnerable populations with impunity. The responsibility to protect doctrine, which emerged around 2000, has been hampered by disagreements in the United Nations Security Council and a lack of political will.