Sin is any failure to perfectly follow God's law.
Sin, according to the Westminster Shorter Catechism is any want of conformity unto or transgression of the law of God.
The Westminster Larger Catechism slightly expands and clarifies, by stating that sin is any want of conformity unto, or transgression of, any law of God, given as a rule to the reasonable creature.
The Westminster Confession of Faith treats the matter implicitly: "Every sin, both original and actual, being a transgression of the righteous law of God, and contrary thereunto, does in its own nature, bring guilt upon the sinner, whereby he is bound over to the wrath of God, and curse of the law, and so made subject to death, with all miseries spiritual, temporal, and eternal."
The first twenty questions of the Heidelberg Catechism (link) do a great job of discussing sin, its effect, and its cure. In fact, any Roman Catholic who concerns Reformed Theology to be in error should be prepared to respond to those twenty questions and answers.
Why belabor this point? Because there are certain folks running around these days claiming that sin is only voluntary violation of the law of God: i.e. that only conscious, volitional infractions are sins, namely that one can only sin by choosing to sin.
Before we prove that point, it is important to point out the voluntary, intentional sins are still sins and they are particularly heinous sins. We will return to this in a moment.
The moral law of God, as summarily comprehended (i.e. summarized) in the ten commandments, does not include a "volitional" component, especially as some of its commands are positive. More especially, the most famous summary of the moral law:
Deuteronomy 6:5 And thou shalt love the LORD thy God with all thine heart, and with all thy soul, and with all thy might.
Leviticus 19:18 Thou shalt not avenge, nor bear any grudge against the children of thy people, but thou shalt love thy neighbour as thyself: I am the LORD.
35Then one of them, which was a lawyer, asked him a question, tempting him, and saying, 36Master, which is the great commandment in the law? 37Jesus said unto him, Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind. 38This is the first and great commandment. 39And the second is like unto it, Thou shalt love thy neighbour as thyself. 40On these two commandments hang all the law and the prophets.
Both of these are phrased positively, namely by stating what we must do, not what we are forbidden to do. Failure to do what are supposed to do can be attributed to many factors, but it should be readily apparent that one can fall short of loving God or of loving one's neighbor without voluntary act: in fact, failure to act may itself be a violation of the law of God, where loving God or one's neighbor requires action.
A second argument against a requirement that sin be positively voluntary are the references to sins of ignorance in Scripture:
Numbers 15:28 And the priest shall make an atonement for the soul that sinneth ignorantly, when he sinneth by ignorance before the LORD, to make an atonement for him; and it shall be forgiven him.
Deuteronomy 19:4 And this is the case of the slayer, which shall flee thither, that he may live: Whoso killeth his neighbour ignorantly, whom he hated not in time past;
1 Timothy 1:13 Who was before a blasphemer, and a persecutor, and injurious: but I obtained mercy, because I did it ignorantly in unbelief.
The first verse identified above makes it clear that sins done in ignorance are still sins, are still in need of atonement, and consequently are something that can be forgiven. The second verse shows the breadth of ignorant sin, in that it encompasses the unintentional. The third verse provides confirmation that sins of ignorance are still justly to be punished, and that mercy is the way of escape from punishment for such sins.
Some may object that sins are described as willing or willful (Hebrews 10:26 For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins, ). It should be understood, however, that willing sin is more heinous than done in ignorance, thus premeditated killing (punished by execution) is more serious than accidental killing (punished by exile to a city of refuge). Thus, some sins are more heinous than others, with willful sins being more heinous than ignorant sins.
One non-Calvinist poster named "FreeGrace" has challenged not that God prohibits sin, but has suggested that God does not require perfect obedience. On the contrary, yes, God requires perfect obedience.
Genesis 17:1 And when Abram was ninety years old and nine, the LORD appeared to Abram, and said unto him, I am the Almighty God; walk before me, and be thou perfect.
Deuteronomy 18:13 Thou shalt be perfect with the LORD thy God.
Matthew 5:48 Be ye therefore perfect, even as your Father which is in heaven is perfect.
James 2:10 For whosoever shall keep the whole law, and yet offend in one point, he is guilty of all.