Sin is any want of conformity unto, or transgression of, the law of God.
Westminster Shorter Catechism
Sin is an offense against reason, truth, and right conscience; it is failure in genuine love for God and neighbor caused by a perverse attachment to certain goods. It wounds the nature of man and injures human solidarity. It has been defined as "an utterance, a deed, or a desire contrary to the eternal law."
Catholic catechism
Ooo, the catechism!