Sin is disobedience to any commandment that applies to me...a transgression of God's moral law.
I used to be a Sabbath/festival/clean and unclean meat observer, and those people would tell you that sin is transgression of the Old Covenant, Ten Commandments, or their patchwork quilt of Old Covenant commandments. I am not an Israelite that was a party to the Old Covenant, and it's no longer in effect anyways (Acts 15, II Corinthians 3, Hebrews 8 and 9, Ephesians 2:13-15, and Galatians 3 and 4).
Of course, they have their rescue devices regarding their assertions, but they really can't get around that.
The Old Covenant was A revelation of God's character in a juvenile way (it was meant to be temporary and preparatory, almost like kindergarten is preparatory and temporary for more mature mature teaching), and there are moral components within the Old Covenant, but not every commandment in the Old Covenant was moral in nature. Many were ceremonial or ritualistic. Many were shadows that were fulfilled in Christ, who is the reality (Colossians 2:16-17, Hebrews 4:9-10 indicates that the Sabbath and festivals were part of those). A Spirit-filled Christian can read the Old Covenant and see spiritual and moral principles underneath the specific applications given to Israel, though, and discern moral direction from them.
Anyways, when I think about the definition of sin, my focus is on Jesus Christ and his perfect holiness. He was the full revelation of God's holiness, and not the Old Covenant law. When it gets right down to it, though, God's holiness is the standard. The Sermon on the Mount mentions this in Matthew 5:48. Any deviation from the perfection of God is sin.
Some will read this and claim that it means "maturity" rather than perfection. They will claim that they meet the standard of Matthew 5:48. They are fooling themselves; they don't meet the standard at all, and they are circumventing Christ's teaching in this regard. Christ's sermon on the mount was for the express purpose of showing the Jews how far they fell short. That realization should have caused them to accept their need for him and his grace and mercy and to realize their hopeless reliance upon that for salvation.
I really hate it when lawkeepers and sinless perfectionists claim that they meet God's standard of perfection. It makes me sick to be honest. It circumvents their realization that they are not perfect in the sight of Almighty God, which negates their need for the continual mercy and grace God provides. It can also cause them to deny the concept of imputed righteousness which is fundamental to the gospel message.
While the Sermon on the Mount does prescribe the ideal behavior that Christians need to seek to implement in their daily life through the transforming power of the Holy Spirit, many "lawkeepers" and all "sinless perfectionists" are fooling themselves if they think they are meeting that standard and are not in need of God's continual grace and mercy, and the imputed righteousness of Jesus Christ. They also fail to realize that the Law (and the Sermon on the Mount) was meant to condemn the Jews and to force them to realize their need for Christ and grace through Him. Read II Corinthians 3 in light of this. The Law had a ministry of condemnation that a lot of "lawkeepers" deny.
God bless you
