[Matthew Shea]

Wow. I'm amazed by some of these responses.

In my experience, sin is definitely very real in the world and the definition is really quite simple. Anything that interferes with your relationship with God (assuming belief) or harms your fellow humans is sinful.

Of course, as was pointed out above, you have to take the whole context into account when judging something as sinful. There's nothing I can imagine to excuse, for example, the killing of 6 million Jews, but I think I would be justified in killing someone invading my home, knowing my life was in imminent danger and I had no other choice. You can still argue, however, that in the latter case, I still committed sin, but my action was justified given that my survival was at stake. I should point out that just because something is justified, doesn't make it "good," but it can lessen the responsibility for and/or consequences of a sinful act.