sin
Any failure to conform to the moral law of God in act, attitude, or nature.1

• From scripture: 
  

  Everyone who makes a practice of sinning also practices lawlessness; sin is lawlessness. (1 John 3:4 ESV)

  …for all have sinned and fall short of the glory of God…. (Romans 3:23 ESV)

• From the Westminster Larger Catechism:

  Question 24: What is sin?

  Answer: Sin is any want of conformity unto, or transgression of, any law of God, given as a rule to the reasonable creature.

• From Systematic Theology by Louis Berkhof:

Sin always has a relation to God and His will. The older dogmaticians realized that is was impossible to have a correct conception of sin without contemplating it in relation to God and His will, and therefore emphasized this aspect and usually spoke of sin as “lack of conformity to the law of God.” This is undoubtedly a correct formal definition of sin. But the question arises, Just what is the material content of the law? What does it demand? If this question is answered, it will be possible to determine what sin is in a material sense. Now there is no doubt about it that the great central demand of the law is love to God. And if from the material point of view moral goodness consists in love to God, then moral evil must consist in the opposite. It is separation from God, opposition to God, hatred of God, and this manifests itself in constant transgression of the law of God in thought, word and deed. The following passages clearly show that Scripture contemplates sin in relation to God and His law, either as written on the tablets of the heart, or as given by Moses….

• From What Is Sin? by J. Gresham Machen:

What, then, is sin? We have said what it is not. Now we ought to say what it is. Fortunately we do not have to search very long in the Bible to find the answer to that question. The Bible gives the answer right at the beginning in the account that it gives of the very first sin of man. What was that first sin of man, according to the Bible? Is not the answer perfectly clear? Why, it was disobedience to a command of God. God said, “Ye shall not eat of the fruit of the tree”; man ate of the fruit of the tree: and that was sin. There we have our definition of sin at last.

“Sin is any want of conformity unto, or transgression of, the law of God.” Those are the words of the Shorter Catechism, not of the Bible; but they are true to what the Bible teaches from Genesis to Revelation. The most elementary thing about sin is that it is that which is contrary to God’s law. You cannot believe in the existence of sin unless you believe in the existence of the law of God. The idea of sin and the idea of law go together.

That being so, I ask you just to run through the Bible in your mind and consider how very pervasive in the Bible is the Bible’s teaching about the law of God. We have already observed how clear that teaching is in the account which the Bible gives of the first sin of man. God said, “Ye shall not eat of the fruit of the tree”. That was God’s law; it was a definite command. Man disobeyed that command; man did what God told him not to do: and that was sin. But the law of God runs all through the Bible. It is not found just in this passage or that, but it is the background of everything that the Bible says regarding the relations between God and man.

…

This law is grounded in the infinite perfection of the being of God Himself. “Be ye therefore perfect,” said Jesus, “even as your Father which is in heaven is perfect” (Matthew 5:48). That is the standard. It is a holy law, as God Himself is holy. If that be the law of God, how awful a thing is sin! Not an offence against some rule proceeding from temporal authority or enforced by temporal penalties, but an offence against the infinite and eternal God!