What is sin?
The Greek word translated "sin" literally means "missing the mark." The picture is of an archer aiming at a target and not hitting it. A more modern picture might be of the golfer who tees off and instead of landing on the fairway slices into the trees.
The Bible itself generally assumes that one understands that sin is disobedience to God. James gives a partial definition. "Therefore, to him that knows to do good and does not do it, to him it is sin." (James 4:17) This is only a partial definition, however, because even under the Law of Moses there were sacrifices required for those who unknowingly sin.
In Romans, chapters 1 through 3, Paul uses the similar terms "ungodliness" and "unrighteousness." The idea he presents is that sin is defiance of God and that it is injustice or wrongdoing. These ideas fit easily into a general definition of sin.