What are "crimes against humanity"?

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Just wondering what the real definition of this term is. Anyone know? What kinds of things would be considered 'crimes against humanity'? WikipediA lists a bunch of examples and one says: "Other inhumane acts of a similar character (to the others listed with this one)intentionally causing great suffering, or serious injury to body or to mental or physical health". Hmmmmmmmmmmmmmmm.....