It is the mass extermination of a whole group of people, an attempt to destroy an entire group and wipe them out of existence.
But at the heart of this simple idea is a complicated tangle of legal definitions.
This has led to conflicting views on when a mass killing, or forced movement, of people can be called genocide.
There are people who say that there was only one genocide during the last century.
Others say there were at least three, possibly more.
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