In Abrahamic contexts, sin is the act of violating God's will.
Sin can also be viewed as anything that violates the ideal relationship between an individual and God; or as any diversion from the ideal order for human living. To sin has been defined as "to miss the mark".
Sins fall in a spectrum from minor errors to deadly misdeeds. Catholicism regards the least corrupt sins as venial sins which are part of human living and carry immediate consequences on earth, and, if unrepented for, more painful purgation, assuming the person is destined to heaven, as it is written in the formation letter "Purgatory", "most of the early Fathers of the Church speak of a cleansing fire, though we cannot tell whether this means actual or spiritual fire."
Conversely, sins of great evil are mortal sins which bring the consequence of hell if they are not addressed either through an act of perfect contrition or going to confession about them.