Sin is any thought, word, or deed in opposition to the perfect law of God.

"Everyone who sins is breaking God’s law, for all sin is contrary to the law of God."
-1 John 3:4 NLT

"And then he added, “It is what comes from inside that defiles you. For from within, out of a person’s heart, come evil thoughts, sexual immorality, theft, murder, adultery, greed, wickedness, deceit, lustful desires, envy, slander, pride, and foolishness. All these vile things come from within; they are what defile you.”"
-Mark 7:20-23 NLT