Sin in the Orthodox Christian understanding is "missing the mark" (the literal meaning of the Greek word for sin, hamartia), falling short of the glorious purpose for which God created mankind. It is also understood as separation from God, since intimate communion with God is the normal state of mankind from which most people have fallen. Sin is imperfection, anything which fails to live up to the fullness of life in Christ for which man was created.
The Bible sometimes uses legal metaphors to refer to sin, likening it to crime, that is, crime against God's law. For Orthodox Christianity, while making use of legal imagery, the more dominant imagery used for sin is also drawn from Scripture, and that is that sin is a kind of disease, an affliction for which salvation is the cure.