Hi!

There is a definition of genocide that has been adopted by the United Nations General Assembly. It was adopted back in 1949, actually, and is called the Convention on the Prevention and Punishment of the Crime of Genocide (otherwise known as the 1949 Genocide Convention). Unlike other phrases like "terrorism" which have yet to be perfectly defined with broad consensus, genocide as a phrase is fairly standardly defined. For example, the 1949 Convention defines genocide in Article 2, saying:

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.

This definition is held fairly standard throughout time as well. For example, the International Criminal Tribunal for the Former Yugoslavia (ICTY) uses that definition, as does the International Criminal Tribunal for Rwanda (ICTR). It is the standard, internationally used definition, and like any statute is subject to variations in interpretation based on acts committed. But it is fairly clear and straightforward.

Hope that helps!