What is sin?
First; the original sin:
God created Adam and Eve in a state of holiness, without sin (Gen. 2:25). Through their own free will, they disobeyed God. Their disobedience brought death and corruption into their nature and consequently the whole world. The corruption is inherited by all human being and is called "Original Sin" (see Rom 5:12-19).
The mystery of Baptism cleanses us from original sin. It restores us to original righteousness and fellowship with God (see Acts 2:38; 22:16). After Baptism we are still left with a free will and a certain moral and spiritual weakness. The only child who was conceived without original sin is the Lord Jesus Christ (2 Cor. 5:21).
Second; our daily sin:
Sin is the breaking of God's law. If God says "Do not lie" and we lie, then we have broken His law and sinned. When we sin, we offend God because it is His law that we have broken. Also, the reason God says to not lie, not cheat, etc., is because these laws reflect the moral purity of His nature. Therefore, the Law is a reflection of the character of God. It is wrong to lie because God cannot lie. It is wrong to steal because God cannot steal, etc. The moral law is not arbitrary. It is based on God's holiness. The word "cannot" does not suggest the limitation of God's ability. It implies that this action is against His nature. It is as if we say that light can not be darkness or life can not be death.
The law, then, is a standard of moral purity. The Bible contains the Law of God. It is a perfect standard because it is God's standard. When we fail to keep the law, we sin. When we sin, we offend God. This offence against God results in a judgment. Laws are laws because they have penalties. There is no law without a penalty. Therefore, breaking God's law brings judgment which is separation from God. "But your sins have made a separation between you and your God," (Isaiah 59:2). And, "the wages of sin is death..." (Rom. 6:23). So, to sin, to break God's law, results in judgment. The judgement is known as damnation which is the righteous judgment of God upon the sinner. If God did not judge the sinner, then He is not upholding His holiness and he would be allowing sinners to go unpunished.
The Lord Jesus Christ came to take our place and die for our sins (1 Pet. 2:24). This means that the Lord Jesus bore our sin in His body on the cross and paid for them. He took the judgment upon himself. This further means that anyone who believe in what the Lord Jesus did on the cross will have his sins removed; he will be saved from God's future judgment. Of course this will only take place when we repent and confess our sins.
So, replying to the question "What is sin?" is best answered by saying that it is breaking God's law.