It is not simply "doing something wrong," or "making a mistake"
It is a product of man's freedom
It "hates the light" (cf. John 3:19; John 2:9), and we, sinners all, are often ashamed to take it seriously
It is any deed, word, or desire against eternal law (St. Augustine)
It is the voluntary transgression of the law of God (St. Thomas Aquinas)
It is turning away from God
It is a moral attitude, power action, or refusal to act that leads us into evil, and alienates/separates us
It is merely breaking some impersonal law in a book
It is a guilt feeling or something we cannot avoid