Q: What is sin?
A: Sin is any want of conformity unto, or transgression of, the law of God.

1 John 3:4. Whosoever committeth sin transgresseth also the law: for sin is the transgression of the law.
James 4:17. Therefore to him that knoweth to do good, and doeth it not, to him it is sin.
Romans 3:23. For all have sinned, and come short of the glory of God.

Westminster Project - See John Flavel's Exoposition of the Shorter Catechism for more info