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.
The Bible says that the sins of the parents are visited onto the children, unto the seventh generation. This could be a social stigma thing; for example, if a man were to commit treason in the ancient Roman empire, then he would be put to death, along with his wife and children, and any other close members of his family (for example, his brothers and sisters, their partners and their children) and the stain would remain on the family name for generations.
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. If any one of these elements are missing, then one is not considered to be doing "a sin".
Sin is that choice you make which severs you from relationship with your deity.