The definition of genocide

Just because:

In the present Convention, genocide means any of the following acts committed with 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.

From the 1948 Convention on the Prevention and Punishment of the Crime of Genocide. The Convention was ratified by the US in 1988(!) with several understandings (essentially our interpretation of certain words and phrases):

Understandings:
"(1) That the term `intent to destroy, in whole or in part, a national, ethnical, racial, or religious group as such' appearing in article II means the specific intent to destroy, in whole or in substantial part, a national, ethnical, racial or religious group as such by the acts specified in article II.
(2) That the term `mental harm' in article II (b) means permanent impairment of mental faculties through drugs, torture or similar techniques.
(3) That the pledge to grant extradition in accordance with a state's laws and treaties in force found in article VII extends only to acts which are criminal under the laws of both the requesting and the requested state and nothing in article VI affects the right of any state to bring to trial before its own tribunals any of its nationals for acts committed outside a state.
(4) That acts in the course of armed conflicts committed without the specific intent required by article II are not sufficient to constitute genocide as defined by this Convention.
(5) That with regard to the reference to an international penal tribunal in article VI of the Convention, the United States declares that it reserves the right to effect its participation in any such tribunal only by a treaty entered into specifically for that purpose with the advice and consent of the Senate."

Whether it matters or not for policymaking is another debate all together, but this is the legal definition.