What is the clearest definition of sin?
1 John 3:4 says, "Sin is transgression of the law." This refers to sins of commission.
James 4:17 says, "One who knows the right thing to do and does not do it, to him it is sin". This refers to sins of omission.
But the clearest definition of sin is in Romans 3:23: Sin is "coming short of God's glory". Sin is not just breaking the Ten Commandments, nor even coming short of the standard Jesus laid down in the Sermon on the Mount (Matthew 5 to 7). It is more than these. It is coming short of the glory of God.
What is the glory of God? "The Word became flesh, and we saw His glory…full of grace and truth" (John 1:14). The glory of God was seen in the life of Jesus Christ. So when we put these two verse together (John 1:14 and Romans 3:23), we see that sin is anything unlike Christ. Who then can say that he is totally free from sin? No-one. Because none of us have become totally like Christ.
It is those who lower their definition of sin who imagine that they are totally free from sin. Pass-marks in God's kingdom is not 40% but 100%. Anything less than total likeness to Christ is sin. Many believers feel that they were not as bad sinners as some others. But that would be like a student who got 2% in an examination considering himself better than another student who got 1%. But both of them failed! Even if you managed to get 99% in God's kingdom, you would be in the same class as the one who got zero. "The person who keeps every law of God but makes one little slip is just as guilty as the person who has broken every law there is" (James 2:10 - Living).
Having known thousands of Christians in my life, I have come to the conclusion that the fundamental problem with the vast majority of them is that they have never seen themselves as filthy, good-for-nothing sinners in God's eyes. They will acknowledge that they are sinners but not as bad as some others. This hinders them from enjoying the full power of the Holy Spirit for a full salvation in their lives.
Many people avoid sin, fearing the punishment they will get. But when God sees someone sin, He thinks of the glory he is missing. There is a glory that you will miss throughout eternity if you tolerate any sin in your life. The measure in which you tolerate sin in your life will be the measure of the glory that you will miss.