When characters say something that's the opposite of what they mean, they're using

The verbal form of irony is sarcasm, which is when someone says the opposite of what he or she means or says something out of character, usually to emphasize the absurdity of something or to convey the opposite point. ... The other use for irony us to convey something opposite to what you're actually saying.

When characters say something that is the opposite of what they mean they are using sarcasm. 

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