Sin is defined as an offense against reason, truth and right conscience. It is said to hurt both the nature of man and human solidarity. It is a contradiction to the Eternal Law as well.

When we sin, we put both our will and desires before God's. Sin is also a rejection of right conscience and a refusal of God's gifts. People minly think of sins as deeds, actions, or courses. But they can also be cruel thoughts or words against God.

Definition of Sin in Catechism