You might be thinking of fair use, but that's an affirmative defence. Every time someone has copied someone elses artwork and modified it into a meme, that's co

You might be thinking of fair use, but that's an affirmative defence. Every time someone has copied someone elses artwork and modified it into a meme, that's copyright infringement and remains so even if is eventually ruled as fair use. If you make a fair use claim, you don't deny infringement, you make the claim that you were allowed to infringe.