1 John 3:4
Whosoever committeth sin transgresseth also the law: for sin is the transgression of the law.
Which law? The Ten Commandments only or all of the laws handed down to Israel? Which begs another question; is everybody to follow the laws handed down to Israel, both Jews and Gentles? This was a question that was discussed in the book of Acts.
Acts 15:7-11
And when there had been much disputing, Peter rose up, and said unto them, Men [and] brethren, ye know how that a good while ago God made choice among us, that the Gentiles by my mouth should hear the word of the gospel, and believe. And God, which knoweth the hearts, bare them witness, giving them the Holy Ghost, even as [he did] unto us; And put no difference between us and them, purifying their hearts by faith. Now therefore why tempt ye God, to put a yoke upon the neck of the disciples, which neither our fathers nor we were able to bear? But we believe that through the grace of the Lord Jesus Christ we shall be saved, even as they.
The answer they agreed upon was this:
Acts 15:19-20
Wherefore my sentence is, that we trouble not them, which from among the Gentiles are turned to God: But that we write unto them, that they abstain from pollutions of idols, and [from] fornication, and [from] things strangled, and [from] blood.
And we read in another place:
Romans 6:14
For sin shall not have dominion over you: for ye are not under the law, but under grace.
If sin is transgression of the law and we are no longer under the law what then is sin now?
James 4:17
Therefore to him that knoweth to do good, and doeth [it] not, to him it is sin.
Romans 2:12-15
For as many as have sinned without law shall also perish without law: and as many as have sinned in the law shall be judged by the law; (For not the hearers of the law are just before God, but the doers of the law shall be justified. For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves: Which shew the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another)
Sin, I believe, is disobeying that inside us that tell us right from wrong. What is inside us and guides us should be the Holy Spirit of God if we be in Him.