What defines something as dark magic?

What defines a curse or spell as dark magic in the Potterverse?

I found this quote

Harry saw a clean, gaping hole where George’s ear had been.



‘How is he?’



Mrs Weasley looked round and said, ‘I can’t make it grow back, not when it’s been removed by Dark Magic. But it could have been so much worse ... he’s alive.’



_{Deathly Hallows - pages 64 and 66 - Bloomsbury - chapter 5, The Fallen Warrior}

and was wondering what makes something dark, or dark magic.