Sin in the bible, is, as many say, acting against God's law. Sin comes from the Hebrew hattah, to miss the signpost.
God gives signposts, that is to say, he speaks to man through prophets, teachers and law-givers, in order that man might live a life of happiness and return to God at the end of life. This is the biblical teaching.
So sin in the Bible is about missing the mark, the sign, and the people of the Bible, the Israelites, experience God as a jealous God who will bring his people back to him when they stray. This is the whole kernel of the Law and the Prophets. It is the Prophets, Ezekiel and Jeremiah, Hosea and Micah, Amos and Isaiah, who make this point again and again.
God calls out with love, God sends signs and wonders, speakers and teachers, yet, man builds the Tower of Babel, whores after Baal and other foreign gods made of clay and stone, and ignores the Code, the Ten Commandments and the divine law written on their hearts. This was the message of Jeremiah, and the Book of Deuteronomy, "I will give them a new law, a Law written on their hearts".
Jesus taught that sin can be forgiven, and to forgive is a sign of divine love in the heart of man and woman.
From a spiritual point of view, sin is that which separates you from your deity or God. Sin severs that relationship. Brings it to a halt.
From a behavioural point of view, sin is an act which transgresses a moral code specified or given by a deity or God.
From the point of view of a Christian, (specifically Catholicism) sin involves will, i.e., decision or choice to act or behave in a certain manner which transgresses the moral codes of Christianity. There are three philosophical criteria which must exist before one is considered to be sinning. There must be full knowledge, full consent to act, and an intention to act. Bold text