To me, a genocide is hard to define, but there are a few things that have to happen in order for something to be a genocide.
First, a lot of people have to be killed. You can't have a genocide of just a few people.
Second, the people killed have to be of some particular race or ethnic group that is different than that of the people doing the killing. It doesn't have to be a difference that makes sense, but as long as they see it as a difference and it is racial/ethnic (not political), it can be a genocide.
Third, there has to be an attempt to kill the whole group of people.
Fourth, the killing must be based on the victims' racial/ethnic status.