According to the United States Holocaust Memorial Museum's website,
Genocide is a term created during the Holocaust and declared an international crime in the 1948 United Nations Convention on the Prevention and Punishment of the Crime of Genocide. The Convention defines genocide as any of the following acts committed with the intent to destroy, in whole or in part, a national, ethnical, racial or religious group, as such:
a. Killing members of the group;
b. Causing serious bodily or mental harm to members of the group;
c. Deliberately inflicting on the group conditions of life calculated to bring about its physical destruction in whole or in part;
d. Imposing measures intended to prevent births within the group;
e. Forcibly transferring children of the group to another group.
The specific "intent to destroy" particular groups is unique to genocide. A closely related category of international law, crimes against humanity, is defined as widespread or systematic attacks against civilians.
You can learn more about genocide, including a timeline of the term, by clicking here.