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.