Sin is any offence which is against God.
And we know exactly what offends God because He gave us the Law, the 10 Commandments.
If we break God's Law, we are offending God.
"Whosoever committeth sin transgresseth also the Law: for sin in the transgression of the Law."
1 Jn. 3:4
Paul says: "... Is the Law sin? God forbid. ..."
Rom. 7:7
"... the Law is holy, and the Commandment holy, and just, and good."
Rom. 7:12
And Paul was:
"... a keeper of the Law."
Acts 21:24
Sin is the breaking of the Law, the breaking of the 10 Commandments.
Per God's Word.