Well if it's a discussion you're after (and I realize I am going to be ninja'd into forever while I type this out)
The definition of sin that I am most familiar with is deed and thought that doesn't line up with God's will. Deed is, of course, the most outwardly visible of these. It is also the least important of these. Because it is impossible to live without having been completely perfect, these do exist for everyone. Every single person. But they are still the least important. Thought is a more complex and less naive view of things, but preachers long ago found that when you try to pitch that to congregations, you only reach about 20% of the audience, even if they're going along with everything you're saying about deed. So, not as much emphasis is placed on it. Regardless, let's delve into these.
I don't believe in this particular list, but let's go for it because everyone will recognize it: wrath, avarice, sloth, pride, lust, envy, and gluttony. Thought is more important because deeds are temporary things. You get mad at your friend so you hit him, but after it's done you two work it out, and even though you've sinned and you hurt someone, everything is fine isn't it? Well, you've still sinned, but that's a deed. But what if you did it a lot? What if you constantly gave into
wrath? Well, you probably wouldn't have friends for long, and, you know, God's just as uncomfortable watching you blow up at other people he's quite fond of and it does make everything very awkward when he's trying to like you too, so ...
Then, let's distill all of this down to its very essence, because even with all the thought sins, there's really only one in there that's really bad. Pride. Everything else on that list is something you can be ashamed of having in a reasonable society. Every other thing on that list is something that can pass though a person, and if they're called on it it's fine. Pride will look you in the eye if you tell it that it's wrong and go "fuck you, you're not my dad, and even if you are, fuck you harder!" It's a zero sum sin. It's confrontational. It's the king on the mountain who says that he's worked hard enough to deserve his place, and everyone else is inferior. It's the man on the street who tosses your money back in your face.
It's this youtube video. It's the sin that you have that will tell you that you're all alone in the world and no one's going to look out for you. It's the sin that closes you off, and when you see it other people it makes you want to punch them in the face, but you can almost never see it in yourself.
Aside: since it's rather confusing, there
are a couple of definitions of pride, and the one I'm referring to is
most certainly not the one that you use in the context of "I'm proud of who I am!" or "I'm proud of you!" That's the pride of satisfaction about something you like. Self-esteem, satisfaction in a job that you've done, satisfaction about your own identity, and the identity of the community that surrounds you, these are important and good things. The pride I refer to as a sin is when the ego overrides reality, and you isolate everything good to yourself without consideration of others.
Anyway, I realize that this is not the definition that the average Christian is going to give you, but after 2,000 years of discussion and divergence on this topic, this is the definition that I've heard that resonates with me the best. Cheers.