Sin is a term used mainly in a religious context to describe an act that violates a moral rule, or the state of having committed such a violation. The English word sin was originally an archery term. The distance from the center of the bullseye to the point where an arrow struck is known as the 'sin of the arrow'. Sin is often used meaning an action thought of as wrong or prohibited however in some religions (most notably Christianity), sin or sinning is something that is an action or a state of mind.
In monotheistic religions, the code of conduct is determined by God. Colloquially, any thought, word, or act considered immoral, shameful, harmful, or alienative might be termed "sinful".