From YOUCAT, page 315:
315: What is a sin in the first place?
A sin is a word, deed, or intention by which man deliberately and voluntarily offends against the true order of things, as God’s loving providence has arranged them. [1849-1851, 1871-1872]
To sin means more than to violate some rules about which men have agreed. Sin turns freely and deliberately against God’s love and ignores him. Sin is ultimately “love of oneself even to comtempt of God” (St. Augustine), and in the extreme case the sinful creature says, “I want to be like God” (see Gen. 3:5). Just as sin burdens me with guilt, wounds me, and by its consequences ruins me, so too it poisons and damages the world in which I live. It becomes possible to recognize sin and its seriousness by drawing near to God.