An antonym for the word analogy?
I don’t think there is one. An analogy is a special kind of abstract relationship. An antonym, or opposite, means something at the farthest possible extreme away–something at the opposite end of the continuum. But something either is an analogy or is not (and that may even be a matter of interpretation or opinion). There is no meaning to the idea of being the farthest possible thing from an analogy.