The entire narrative of the Bible has to do with sin, so there’s not really one specific verse that says, “This is the definition of sin.” The very first time we encounter the word “sin,” is when God says to Cain, “If you do not do well, sin is lurking at your door; its desire is for you, but you must master it.” (Genesis 4:7) However, the first time we encounter an act of sin is when Adam and Eve eat the fruit of the Tree of the Knowledge of Good and Evil, their one and only rule (Genesis 3:6). The rest of the Bible goes on to talk about this sin problem of ours.
The best simple definition of sin I’ve heard is that sin is trusting in created things rather than God. That created thing is often ourselves. Whenever we do this, we do so to the neglect of everything God loves. We mistreat others or don’t help others. The Law of Moses came about to expose us for the sinful beings we are, to expose our trust in created things over our trust in God. That is why the Law doesn’t save us from sin, because its job is to point out our sin.
That’s why we have Jesus. John says, “For the Law was given through Moses; grace and truth came through Jesus Christ.” (John 1:17) With sin came death. A world of sin where we could live forever would be Hell on Earth. But through Jesus’ death and resurrection, we are promised a life freed from the power of sin and an unbroken relationship with God, who created us and has determined that all of this is worth it so that we could experience love for God’s creation, each other, and know the ultimate love that comes from God alone.