Sin is forsaking fellowship with God in order to pursue your own pleasures.

Against you, and you alone, have I sinned; I have done what is evil in your sight. You will be proved right in what you say, and your judgment against me is just. Create in me a clean heart, O God. Renew a loyal spirit within me. Do not banish me from your presence, and don’t take your Holy Spirit from me. Restore to me the joy of your salvation, and make me willing to obey you. Psalm 51