In Chapter 1 of his book of this name, Cornelius Plantinga, Jr. defines sin (see also part 5):
All sin has first and finally a Godward force. Let us say that a sin is any act -- any thought, desire, emotion, word, or deed -- or its particular absence, that displeases God and deserves blame. Let us add that the disposition to commit sins also displeases God and deserves blame, and let us therefore use the word sin to refer to such instances of both act and disposition. Sin is a culpable and personal affront to a personal God. p. 13
In the footnotes, Plantinga distinguishes thoughts, desires, emotions, and words from "deeds," though all come under the umbrella of sinful acts. He also explains that his definition is criteriological as opposed to ontological: it "tells us what is sinful but not what is sin." What sin is, he says, is the power that corrupts human thoughts, desires, etc., displeasing God and making humans guilty.
Plantinga rightly defines sin as a condition or state of mind which results in sinful acts, which are also sin. There seems to be a tendency to think of sin only in terms of acts rather than as the rebellious, prideful, fearful, cowardly, hateful, self-pitying, mistrusting-of-God attitudes that lead to either commission of wrong behavior or omission of right behavior. He also stresses the personal nature of sin against a personal God, i.e., a God who is concerned with us as persons, personally -- a God whose business it is to care about our business -- indeed, to judge it.
And, just as a wrong against someone is a wrong against them, so sin is against God. We are culpable for such wrongdoing because He, who created us, is also Author of right and wrong. And He has graciously endowed us with the capacity to choose His ways, or not.
