Lets look at the biblical definition
1 John 3:4 Whoever commits sin also commits lawlessness, and
sin is lawlessness.
James 2:11 For
He who said, “
Do not commit adultery,”
also said, “Do not murder.” Now if you do not commit adultery, but you do murder,
you have become a transgressor of the law. You can find the rest of what He said and what He personally wrote here
Ten Commandments
Romans 7:7 What shall we say then?
Is the law sin? Certainly not! On the contrary,
I would not have known sin except through the law. For I would not have known covetousness unless the law had said, “You shall not covet.”
Sin is breaking God's law any of the Ten Commandments
Also anything that is not of faith is sin
Romans 14:23 But he who doubts is condemned if he eats, because
he does not
eat from faith;
for whatever is not from faith is sin.
Regarding actions, Jesus addresses this in Mat 5:19-30 using two of the Ten Commandments as examples. He doesn't even want us to have thoughts that lead to breaking God's law- thoughts of anger that lead to committing murder, thoughts of lust that lead to committing adultery. If our heart changes, our actions are a reflection of this. Hence why He wrote His law in our hearts, right where sin begins. Heb 8:10 we just need to cooperate with Him through love John 14:15-18