is,

Disobedience to the will or commands of God.

Sin includes: "not actions only, but neglect of known duty, all evil thoughts, purposes, words, and desires, whatever is contrary to God's commands or law".
(Quote from Webster's 1828)

For the wages of sin is death; but the gift of God is eternal life through Jesus Christ our Lord.
Romans 6:23

Therefore to him that knoweth to do good, and doeth it not, to him it is sin.
James 4:17