This is a question many people don’t really understand how to answer. Here is a biblical perspective of what SIN really is (what the Bible tells us)…

SIN is refusing to believe God and take Him at His Word.
SIN is a lack of faith- for what is not of faith is sin (Romans 14:23)
SIN is breaking God’s commandments.
SIN is crossing the boundary lines drawn by God.
SIN is going your OWN way, rather than His way.
SIN is knowing to do good and not doing it.
SIN is rebellion against God.

So as we can see there are MANY ways to “miss the mark”. As the scriptures say “All have sinned and fall short of the glory of God”. ~Romans 3:23

“sin is the transgression of the law” (lawlessness) ~1 John 3:4
“If we say we have no sin, we deceive ourselves, and the truth is not in us.”~1 John 1:8-10

For further study check out  What is Sin? and The Definition of Sin.