On antinyms

I've been asked a couple of times what one calls a situation where a word is used to mean the opposite of what it normally means. I've usually interpreted the question to be about euphemisms, where the aim is to obscure or hide a reality. When people talk about 'passing away' instead of 'dying', or 'collateral damage' instead of 'war casualties', the reality stays the same, but the word alters. But an experience this week has made me think that my correspondents might have had in mind a diffe...