His word is always a good place to get answers :-)
The Bible Definition
Many religious leaders teach that Christ died for our sins. They teach that His shed blood cleanses us from sin. They preach about being set free from bondage to sin. They preach about freedom from keeping God’s law. But have you noticed that they never define what sin is—that they never challenge people to look into their Bibles to see how God defines sin?
If they did, they would have to tell you to read I John 3:4, where the Apostle John wrote, “Whosoever commits sin transgresses [breaks] also the law: for sin is the transgression [breaking] of the law.” When you sin, you break the law.
But what law?
The Apostle Paul wrote, “What shall we say then? Is the law sin? God forbid. Nay, I had not known sin, but by the law” (Rom. 7:7). God’s law reveals sin.
But what law was Paul writing about?
“…for I had not known lust, except the law had said, You shall not COVET” (same verse). This is the Tenth Commandment, found in Exodus 20:17 and Deuteronomy 5:21. The law that Paul and John referred to was the Ten Commandments. Both apostles taught that when you break them, you sin.
John also wrote, in I John 5:17, “All unrighteousness is sin.” Since God’s commandments are righteousness (Psa. 119:172), then unrighteousness—sin—must be the opposite. It is breaking God’s commandments.
And for those who think the Commandments were done away with I submit the following.......
Malachi 3:6
For I am the Lord, I change not
That's getting awful close to the mode of thought that Righteousness comes by the law (and imposing the mosaic law [e.g. circumcision] on everyone); it doesn't, because the Law only condemns.
(Romans 3:21)
But now apart from the law the righteousness of God has been made known, to which the Law and the Prophets testify.