Original Sin

Original Sin is the sin of Adam passed on to the rest of the human family.  The reformed crowd speak of this as an inclination to do evil, a propensity to sin, or more often as a moral inability to believe God's goodness and trust in Him.  But this unwittingly forces a dichotomy in the mind of a personal sin and a ruined nature, leading people to think you can have a ruined sinful nature and yet be free of sin- babies being the classic example.  They will sin, but they have not yet. 
The answer to that hazard is thus: immoral inclinations are a sin in themselves, because they are a blight on the perfection of God's design. I'll give a rather crude analogy: God designed the perfect hardware platform and set man up and running, but Adam downloaded new operating instructions that ruined the beauty of the original design. The replication of that ruin is a sin, as the entire system is now less then perfect. 
Babies therefore have already committed a personal sin against God when Adam's nature is inherited.  That inclination to be less than wholly devoted to God is their first sin, or original sin, in addition to being the root of the rest of the sins. Stated another way, it is not merely the source of the sins, but it's existence is itself a sin, an affront to God. The poisoned water handed to the Master is an insult, no less than the very existence of the poisoned well by which we draw it from in the midst of the land He created good.