What is a sin?

In class this past week, we have been discussing the conscience of the character Huck Finn in the novel Adventures of Huckleberry Finn by Mark Twain. The specific passage we have been examining involves the decision Huck has made to steal Jim out of slavery. In his heart, he knows that Jim is one of his best friends, and is a man equal to him. The only problem is that society and his conscience is telling him the opposite. As a result of this, he believes he is committing a sin, and acting against god. Is it really a sin to go against society when society is wrong?
Is it a sin to act irrationally when your beliefs tell you it is right? The Puritan's of early America believed that the numerous people they hung were in fact witches and acting with the devil. Their faith proclaimed that the devil would temp the souls of the "weak," and then Satan himself would act through them. They truly believed that they were in danger, and that the people accused needed to be stopped. They didn't understand that accusing people without a fair trial is unjust. They didn't realize that the rumors may have been false or untrue. Though, did they commit sin? They didn't believe they were doing anything wrong. They were afraid for themselves and their community, but even so, many don't believe that is an excuse for the murders they committed.
Is it a sin to do wrong when you know you won't get caught? Olympic swimmer Michael Phelps was recently spotted smoking what was believed to be the illegal substance cannabis. The swimmer is an Olympic champion, and a role model to people all over the world. He disappointed many when the picture was published. The sad truth is that many people believe what he did wasn't wrong. The mistake instead was letting himself get photographed. He is under the spotlight constantly, and he should know that he cannot create a bad image for himself; especially during a time when he is making millions in endorsement deals. If the picture of Michael Phelps had never been brought to the publics’ attention, would people still think he committed a sin?
What is a sin? This question seems to be asked over and over again, and the answer only seems to be getting more blurred as time passes. Is there a general rule for when something sins or does it differ from person to person? This unspecific conclusion only makes it harder for rightness to be enforced.