Ah- well... Something a person has done that has, directly or indirectly, harmed another, especially in a particularly detrimental way.

...Every sin is not equal, of course. [Ah, she can't get this knight out of a compromised position.] It could be...murder, naturally, but it could be turning a blind eye to the suffering of others. Pushing the blame of something onto someone unrelated. Or not listening to a friend in need. What matters, of course, is that the person who has sinned recognizes and takes actions to make amends or correct this. They acknowledge what they've done, and they seek to move forward.